Off-Grid Barndominium inverter suggestions

SilverIris

Hello everyone. I am a complete newb here when it comes to Solar and Off-Grid matters, but have an excellent understanding of DC power and a solid understanding of the basics of AC power. Before jumping into the nuts and bolts of my questions, here's a little background on me. I've been modding Mazda RX-7s since 2007, including a 20B conversion in 2012. Anyone that knows Japanese cars knows that lots talk about it, some try it and only a handful succeed at it. It takes mastering several disciplines to accomplish, including a fantastic understanding of Watt's Law and Ohm's Law. Now I write maintenance manuals, translated from Japanese.

In terms of A/C, I've already done a complete household install from scratch. To make a long story short, I didn't like how the previous person did it, so I ripped all of it out and started fresh. RX-7 owners are quite picky about wiring and the engineering we put into our cars, and keeping things tidy/organized goes a long way when servicing/modification is desired. The general Modus du Operandi is "Overbuilt and Underworked"

Anyway, I'm designing a Barndominium capable of supporting the average automotive-inclined family and had a few questions about going off-grid with it.

After doing the math, referenced to the US national average for household electrical consumption in 2020 (10,715 kWh, or ~30 KWh per 24 hours), I've determined that 676 Sq.Ft of Solar Panels (assuming 15 watts/Sq.Ft) in full sun will satisfy the need in just under 3 hours. Anything after that would go into the battery bank. Probably overkill, but I thought it wise to account for inefficient conditions such as rain/snow/etc.

Now comes to fun part. Since it's a Barndominium, the "main course" is the Garage. I've designed it to have a 8000 lb 4-post lift because "work smarter, not harder" is my motto in life. Plus, I have a 220v MIG welder too. Welder draws up to 17A (3740w at 220v). Most 4-post lifts run on 110v and use ~20A (2200w), but I want to play it safe and budget for a 220v version in case I find a great deal on a used one in good shape.

As I'll be heating with a Wood Stove and the kitchen will have a Gas Stove, the only "heavy" household devices will be a standard clothes dryer and on-demand water heaters (6000w at 220v). Except for the clothes dryer and the dishwasher running (water heater)

Lift: 2200w (20A @110v)
Welder: 3740w (17A @220v)
Water Heater: 6000w (27A @220v)
Dryer: 6000w (27A @220v)
Well Pump*: 745w (3.4A @220v)

Except when doing Laundry or running the Dishwasher, all of these devices' loads are for 5-10 minutes at most and easily staggered to minimize chances of overload. Everything else is just standard household stuff like a TV, fridge, LED lighting, etc.

My main concern is the Inverter to feed all of this. Going to need a Split Phase Inverter, but trying to get enough RMS/Sustained Wattage is the difficult part. Plus, I've noticed a few things that don't add up right.

For example, this one (https://sungoldpower.com/collections/48v-inverter-charger/products/12000w-peak-36000w-48v-split-phase-pure-sine-wave-solar-inverter-charger) is rated for 12kW sustained, but the AC Output terminals seem quite undersized compared to the DC Input Terminals (M8x1.25 or M10x1.5). Applying Watt's Law and the owners manual, we get 54.5 amps @220v and 10awg wiring intended by the manufacturer. For reference, the average electric stove use 6awg THHN between the breaker panel and its 50A outlet. Trying to force that through 10awg is a fire waiting to happen. Also the 18kW version of this inverter uses the same AC output terminals despite 50% more power being put through them. There is no way those cutesy Euro-style terminal blocks could handle the power and heat being fed through them.

30kW @220v adds up to 136.36A. 1/0 (8.2mm) or 2/0 (9.2mm) cable is the right choice for such cases, and proper Cable Lugs are a necessity for secure connections.

So how does everyone address these issues in an electrically-sound manner?

mike95490

You use 48VDC systems

You use industry standard inverters like the Schneider XW series.  Several can be "stacked" to meet the  load requirements.

You don't cut corners to save $200

Get a generator to run the welder. Really.  Inverters can't easily handle the start-stop of the arc and just go nuts and can fry their guts

Use propane for heating, tankless. What a tankless heater uses, almost requires a dedicated inverter, or learn to enjoy resetting overloads.   Heat pump water heaters are another option.

Mini-split heat pumps can be used effectively with solar, as a backup to wood heat

What is your backup generator plan ?    ( Cloudy days )

Where does your water come from, deep well pump ?  HP ?

BB.

Welcome to the forum SilverIris,

It looks like you have done a lot of thinking about this project. And I do have some questions--Such as (roughly) where the system will be located (look at hours of sun per day by season)...

Also, why? Save money, go green, no utility power nearby, no propane/don't want propane, etc.????

In general, in the USA, solar power is something like 5-10x as expensive as utility power when you take everything into account (batteries typically last 5-10 years, conversion losses require more solar and more battery capacity, etc.).

Just to give you an idea of what an "off grid" system may be sized for:

• 1,000 WH per day (1 kWH)--A cabin--LED LIghts, laptop, cell phone charging, RV water pump
• 3,300 WH per day (3.3 kWH)--A very efficient cabin/home with Refrigerator, solar friendly well pump, washing machine, LED TV, Microwave, etc.
• 10,000 WH per day (10 kWH--Or 300 kWH per month)--A relatively efficient home with natural gas/propane for heating/cooking/hot water
• 30,000 WH per day (30 kWH--Or 900 kWH per month)--A full electric home with electric stove/dryer/hot water and some A/C.

Roughly, the typical North American home uses around 500 - 1,000 kWH per month--And can get upwads of 3,000 kWH per month in Texas with full A/C and electric appliances. (all numbers are very rough).

Normally, for somebody that "needs" to go off grid--Suggest ~3.3 kWH per day as a "target" for a middle size off grid power system that is relatively affordable. Means lots of conservation, Energy Star rated appliances, only using the appliances when needed (turn off when not in use), etc...

You are looking at your loads... Not only "running load", but peak loads (like starting fridge, well pump) and how many hours per day will you be running them. Loads that run for many hours per day can be worse than heavy loads like microwave, water pumping, desktop computer--In terms of sizing solar array and battery bank (i.e., kWatts is a "rate" like miles per hour; and kWatt*Hours is an "amount" like miles driven).

Some examples:

• Microwave: 1,200 Watts * 1/4 hour per day (15 minutes per day) = 300 WH per day
• Laptop computer: 30 Watts * 10 hours per day = 300 WH per day
• Desktop computer: 300 Watts * 10 hourrs per day = 3,000 Watt*Hours per day (3 kWH)
• Refrigerator: 600+ Watts starting, 120 Watts running 50% duty cycle, 500+ Watts defrosting. Efficient fridge is around 1.5 kWH per day

For bigger loads... A 40 Gallon electric water heater rated at 4,500 Watts and 3,493 kWH per year:

• 3,493,000 WH per year / 365 days per year = 9,570 WH per day (9.57 kWH per day)
• 9,565 WH per day / 4,500 Watt heater = 2.1 hours per day for heating element

So while 30,000 kWH per day of energy usage--That is quite a bit of power and would probably drive most of the loads you seem to be planning on... But that will not be a "small" or "cheap" off grid system where I would be "starting" at 3.3 kWH per day system.

To give you an idea of what a "basic system" paper design would look like using our rules of thumbs to give you a quick/rough/first pass system design based using flooded cell lead acid batteries.

First the battery bank... For Flooded Cell Lead Acid batteries, design the bank for 2 days of "no sun" (bad weather) and 50% maximum planned discharge (for longer battery life). This also work relatively well for solar charging (discharging bank every day to 50%, the sun is not "in the sky" enough hours in a day to get back to 100% charge--75% to 100% state of charge is "doable" with solar). If you want to talk about, for example, lithium ion battery bank,
they are "nearly" ideal batteries--But very expensive and can be easy to "murder" if you over/under charge them....

• 30,000 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max planned discharge * 1/48 volt battery bank = 2,941 AH @ 48 volt battery bank

To size the solar array--Two calculations. First is based on size of battery bank... Larger battery bank needs more (average) charging current. Typically solar charging a 5% (for summer/weekend system), 10% minimum for full time off grid, and 13% for typical "cost effective" maximum system...

• 2,941 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 11,267 Watt array minimum
• 2,941 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 22,535 Watt array nominal
• 2,941 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 29,295 Watt array "typical" cost effective maximum

And there is sizing the array based on hours per day of sun by location and season... Say you are near Indianapolis Indiana, fixed array, facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Indianapolis
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 50° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.87	3.46	4.18	4.57	4.80	5.24
Jul	Aug	Sep	Oct	Nov	Dec
5.40	5.15	5.21	4.42	3.05	2.56

Just raw calculations of power needed vs hours of sun per day.... You said 3 hours a day for your planning--In this example, that would suggest that one would need to run a genset during November/December/January (at least) to make up for poor sun and bad weather:

• 30,000 WH * 1/0.52 off grid FLA AC system eff * 1/3.0 Hours of sun per day = 19,231 Watt array for "break even" November

Using your 15 Watts per sq foot--That would not be ~676 sqft of array but:

• 19,231 Watt array / 15 Watts per sqft harvest = 1,282 sqft of array (roughly)

And more potential bad news--The above numbers (hours of sun per day) are based on 20 year averages.... Sometimes you will harvest more sun, other times, harvest less. We suggest that you "plan" on using 65% to 50% of your "predicted harvest" for you "base loads"--Loads such as lighting, refrigerator, business related equipment like computer, welder, etc.). The rest of the loads are ones you can turn off during bad weather (A/C, welder, water pumping for irrigation, clothes washing, etc.) or fire up the genset when needed... So you could end up adding your "base loads together" and multiply by 1/0.65 or 1/0.50 (i.e. 2x) to ensure you have enough solar for poor weather conditions. It all depends on how your "base load" math works out...

As it is, looks like a suggested 19,231 to 22,535 to 29,295 Watt array would be "justifiable"... And >=22,535 Watt array minimum for full time off grid system (10% rate of charge).

That is something like $20,000 panels, $20,000 for racking(?), and $40,000 for battery bank (FLA, Rolls batteries).

Also, remember when you are looking at (for example) an 18 kWatt @ 120/240 split phase AC inverter):

• 18,000 Watts / 240 VAC = 75 Amps

But look at the battery bus current:

• 18,000 Watts * 1/0.85 AC inverter eff * 1/42 volts battery cutoff voltage = 504 Amps on DC Battery Bus

Here is a simplified NEC AWG vs rated current for wiring (seen NEC for all deratings):

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

You can see you are looking at some pretty heavy cabling on the DC side... Even 10% battery charging you are looking at 294 Amps (and I highly suggest battery charging current wiring/circuit breaker/branch circuit ratings have 1.25x NEC continuous current derating)--I.e., heavier current for circuits that carry max rated current for hours at a time, like battery charging.

So--This is sort of a "backwards" design... Highly suggest that you start with measuring your loads first:

https://www.amazon.com/s?k=kill+a+watt+meter (kill-a-watt type meter for measuring 120 VAC plug in loads like fridge&computer)
https://www.amazon.com/s?k=whole+house+energy+meter (whole house/larger/multiple circuit monitoring)

A small to medium size system (like 3.3 kWH per day) system is something that a reasonably competent homeowner can build out themselves with research... a 30 kWH per day system.... For a first timer, that is a lot to bite off.

The above is just the results of a series of guesses and estimates on my part, using standard (reasonably conservative) design rules and non-cutting edge equipment (such as flooded cell lead acid batteries).

And be careful... Besides the ability of a large 48 volt FLA battery bank vaporizing wiring and such, if you have issues (problems charging, etc.), keep a close eye on the battery bank. FLA batteries if not properly maintained and charged can "sulfate" starting in days, and completely dying in weeks to a couple of months. So don't buy batteries and have them sitting around for weeks/months without charging, and have backups available (such as a genset +AC battery charger if solar charging fails and/or you have a week or two of bad weather).

Just to give you an idea, generator charging is typically around 10% to 20% rate of charge. For a large bank like this:

• 2,941 AH * 59 volts charging * 1/0.80 Genset derating * 1/0.9 charger eff * 0.10 rate of charge = 24,100 Watt genset minimum suggested (rough estimate)

I will stop here--I am sure you have lots of questions and would like to "correct" my guesses for a "better" answer.

-Bill

BB.

I was also going to suggest looking at solar hot water--That lends itself pretty nicely to DIY projects and is usually a lot cheaper vs using solar electric... (although, folks have setup electric water heaters with solar to heat water when the batteries are full).

There are lots of ways to approach the problem--And as Mike suggests, not everything works well on solar--At least as a start.

We did have one member here that setup a welding/metal fabrication shop in Hawaii with used forklift batteries an large/cheap AC inverters... So it can be done--Just depends on what your desires are.

-Bill

SilverIris

mike95490 said:

You use 48VDC systems

You use industry standard inverters like the Schneider XW series.  Several can be "stacked" to meet the  load requirements.

You don't cut corners to save $200

Get a generator to run the welder. Really.  Inverters can't easily handle the start-stop of the arc and just go nuts and can fry their guts

Use propane for heating, tankless. What a tankless heater uses, almost requires a dedicated inverter, or learn to enjoy resetting overloads.   Heat pump water heaters are another option.

Mini-split heat pumps can be used effectively with solar, as a backup to wood heat

What is your backup generator plan ?    ( Cloudy days )

Where does your water come from, deep well pump ?  HP ?

Thank you for the prompt response. 48VDC is fine with me, seems like the bigger inverters all use 24 or 48v anyway. A Generator, that would add unnecessary complexity in the manner of converting chemical energy (fuel) to mechanical energy, then to electrical energy. In short, internal combustion engines are ~25% efficient, excluding energy lost as heat when converting from one form to another. Far simpler and more efficient to keep energy in the same form at each step. Plus, I already have two V8s to feed...

The following is not precisely nailed down, but will give you an idea of what I'm aiming for with this endeavor. If something needs adjusted to fit properly, it will be adjusted once I have a better understanding of the feasibility.

The behavior you are referring to is common in vehicles with ridiculously overkill stereo systems to the point of being silly. Simple solution is adding a Capacitor to smooth out the voltage. I'm not sure if it would apply to an AC system though, but I will look into it.

Assuming inrush current is similar to a DC motor such as in the Ford Taurus 2-speed radiator fan, typically 2-3x of rated load for the first second, then settles down to the rated load (17A), it's still well within the parameters of an inverter capable of running the whole house. Realistically, it would take at least a Fluke meter to get an accurate reading when pulling the Mig gun's trigger, but it's safe to say that the transformer inside my Mig welder is capable of "buffering" sufficiently to allow the device to work for a set timeframe (Duty Cycle). This is assuming that any inverter is NOT attempting to react to loads placed on it, similar to how a 130 amp Ford Taurus Alternator behaves (Load Response Controller). In other words, I'm NOT using sleep mode on the inverter when welding.

Also, I've been reading a bit here and there about welding using an inverter. Seems like Low Frequency Inverters handle the characteristics of a Mig welder a lot better than high frequency ones. Sounds like a good starting point.

As for Tankless water heaters, it's all in the plumbing design and packaging. For example, the kitchen will have one under the kitchen sink that feeds both sink and dishwasher. The bathrooms on the 2nd floor will share another dedicated heater for all of their fixtures. This manner of "de-centralizing" means less wasted water waiting for it to heat up. At my old place, I had two $30 Chinese 3000 watt heaters from ebay in series for the bathroom and it was uncomfortably hot if I had the output valve nearly closed (maximum time in the heaters). Most of the time, one heater was plenty for a shower. Now you got me thinking that temperature-adjustable 4500 watt heaters might be a more suitable solution than the On/Off heaters I had before. At the same time, your proposal of heat pump water heaters sounds interesting. I'll be checking into that too.

The backup plan is to have sufficient storage to cover 10 days of zero sun. With the 30kWh figure from above, 300kWh is roughly 6250Ah at 48v. Between this and having a full 24 hours of electrical needs fulfilled in 3 hours of sunlight, I should have more than enough capacity regardless of the circumstances. Think Apollo 13.

Water will be from a well. I calculated for a 1hp (745 watts) pump. If I can use a 1/2hp or 3/4hp one instead, that's even better. Where I'm planning on doing this is near where my dad lived, so I'll have to pay a visit and see how he did his well and pump setup. It will get me in the ballpark.

SilverIris

BB. said:

Welcome to the forum SilverIris,

It looks like you have done a lot of thinking about this project. And I do have some questions--Such as (roughly) where the system will be located (look at hours of sun per day by season)...

Also, why? Save money, go green, no utility power nearby, no propane/don't want propane, etc.????

In general, in the USA, solar power is something like 5-10x as expensive as utility power when you take everything into account (batteries typically last 5-10 years, conversion losses require more solar and more battery capacity, etc.).

Just to give you an idea of what an "off grid" system may be sized for:

• 1,000 WH per day (1 kWH)--A cabin--LED LIghts, laptop, cell phone charging, RV water pump
• 3,300 WH per day (3.3 kWH)--A very efficient cabin/home with Refrigerator, solar friendly well pump, washing machine, LED TV, Microwave, etc.
• 10,000 WH per day (10 kWH--Or 300 kWH per month)--A relatively efficient home with natural gas/propane for heating/cooking/hot water
• 30,000 WH per day (30 kWH--Or 900 kWH per month)--A full electric home with electric stove/dryer/hot water and some A/C.

Roughly, the typical North American home uses around 500 - 1,000 kWH per month--And can get upwads of 3,000 kWH per month in Texas with full A/C and electric appliances. (all numbers are very rough).

Normally, for somebody that "needs" to go off grid--Suggest ~3.3 kWH per day as a "target" for a middle size off grid power system that is relatively affordable. Means lots of conservation, Energy Star rated appliances, only using the appliances when needed (turn off when not in use), etc...

You are looking at your loads... Not only "running load", but peak loads (like starting fridge, well pump) and how many hours per day will you be running them. Loads that run for many hours per day can be worse than heavy loads like microwave, water pumping, desktop computer--In terms of sizing solar array and battery bank (i.e., kWatts is a "rate" like miles per hour; and kWatt*Hours is an "amount" like miles driven).

Some examples:

• Microwave: 1,200 Watts * 1/4 hour per day (15 minutes per day) = 300 WH per day
• Laptop computer: 30 Watts * 10 hours per day = 300 WH per day
• Desktop computer: 300 Watts * 10 hourrs per day = 3,000 Watt*Hours per day (3 kWH)
• Refrigerator: 600+ Watts starting, 120 Watts running 50% duty cycle, 500+ Watts defrosting. Efficient fridge is around 1.5 kWH per day

For bigger loads... A 40 Gallon electric water heater rated at 4,500 Watts and 3,493 kWH per year:

• 3,493,000 WH per year / 365 days per year = 9,570 WH per day (9.57 kWH per day)
• 9,565 WH per day / 4,500 Watt heater = 2.1 hours per day for heating element

So while 30,000 kWH per day of energy usage--That is quite a bit of power and would probably drive most of the loads you seem to be planning on... But that will not be a "small" or "cheap" off grid system where I would be "starting" at 3.3 kWH per day system.

To give you an idea of what a "basic system" paper design would look like using our rules of thumbs to give you a quick/rough/first pass system design based using flooded cell lead acid batteries.

First the battery bank... For Flooded Cell Lead Acid batteries, design the bank for 2 days of "no sun" (bad weather) and 50% maximum planned discharge (for longer battery life). This also work relatively well for solar charging (discharging bank every day to 50%, the sun is not "in the sky" enough hours in a day to get back to 100% charge--75% to 100% state of charge is "doable" with solar). If you want to talk about, for example, lithium ion battery bank,
they are "nearly" ideal batteries--But very expensive and can be easy to "murder" if you over/under charge them....

• 30,000 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max planned discharge * 1/48 volt battery bank = 2,941 AH @ 48 volt battery bank

To size the solar array--Two calculations. First is based on size of battery bank... Larger battery bank needs more (average) charging current. Typically solar charging a 5% (for summer/weekend system), 10% minimum for full time off grid, and 13% for typical "cost effective" maximum system...

• 2,941 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 11,267 Watt array minimum
• 2,941 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 22,535 Watt array nominal
• 2,941 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 29,295 Watt array "typical" cost effective maximum

And there is sizing the array based on hours per day of sun by location and season... Say you are near Indianapolis Indiana, fixed array, facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Indianapolis
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 50° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.87	3.46	4.18	4.57	4.80	5.24
Jul	Aug	Sep	Oct	Nov	Dec
5.40	5.15	5.21	4.42	3.05	2.56

Just raw calculations of power needed vs hours of sun per day.... You said 3 hours a day for your planning--In this example, that would suggest that one would need to run a genset during November/December/January (at least) to make up for poor sun and bad weather:

• 30,000 WH * 1/0.52 off grid FLA AC system eff * 1/3.0 Hours of sun per day = 19,231 Watt array for "break even" November

Using your 15 Watts per sq foot--That would not be ~676 sqft of array but:

• 19,231 Watt array / 15 Watts per sqft harvest = 1,282 sqft of array (roughly)

And more potential bad news--The above numbers (hours of sun per day) are based on 20 year averages.... Sometimes you will harvest more sun, other times, harvest less. We suggest that you "plan" on using 65% to 50% of your "predicted harvest" for you "base loads"--Loads such as lighting, refrigerator, business related equipment like computer, welder, etc.). The rest of the loads are ones you can turn off during bad weather (A/C, welder, water pumping for irrigation, clothes washing, etc.) or fire up the genset when needed... So you could end up adding your "base loads together" and multiply by 1/0.65 or 1/0.50 (i.e. 2x) to ensure you have enough solar for poor weather conditions. It all depends on how your "base load" math works out...

As it is, looks like a suggested 19,231 to 22,535 to 29,295 Watt array would be "justifiable"... And >=22,535 Watt array minimum for full time off grid system (10% rate of charge).

That is something like $20,000 panels, $20,000 for racking(?), and $40,000 for battery bank (FLA, Rolls batteries).

Also, remember when you are looking at (for example) an 18 kWatt @ 120/240 split phase AC inverter):

• 18,000 Watts / 240 VAC = 75 Amps

But look at the battery bus current:

• 18,000 Watts * 1/0.85 AC inverter eff * 1/42 volts battery cutoff voltage = 504 Amps on DC Battery Bus

Here is a simplified NEC AWG vs rated current for wiring (seen NEC for all deratings):

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

You can see you are looking at some pretty heavy cabling on the DC side... Even 10% battery charging you are looking at 294 Amps (and I highly suggest battery charging current wiring/circuit breaker/branch circuit ratings have 1.25x NEC continuous current derating)--I.e., heavier current for circuits that carry max rated current for hours at a time, like battery charging.

So--This is sort of a "backwards" design... Highly suggest that you start with measuring your loads first:

https://www.amazon.com/s?k=kill+a+watt+meter (kill-a-watt type meter for measuring 120 VAC plug in loads like fridge&computer)
https://www.amazon.com/s?k=whole+house+energy+meter (whole house/larger/multiple circuit monitoring)

A small to medium size system (like 3.3 kWH per day) system is something that a reasonably competent homeowner can build out themselves with research... a 30 kWH per day system.... For a first timer, that is a lot to bite off.

The above is just the results of a series of guesses and estimates on my part, using standard (reasonably conservative) design rules and non-cutting edge equipment (such as flooded cell lead acid batteries).

And be careful... Besides the ability of a large 48 volt FLA battery bank vaporizing wiring and such, if you have issues (problems charging, etc.), keep a close eye on the battery bank. FLA batteries if not properly maintained and charged can "sulfate" starting in days, and completely dying in weeks to a couple of months. So don't buy batteries and have them sitting around for weeks/months without charging, and have backups available (such as a genset +AC battery charger if solar charging fails and/or you have a week or two of bad weather).

Just to give you an idea, generator charging is typically around 10% to 20% rate of charge. For a large bank like this:

• 2,941 AH * 59 volts charging * 1/0.80 Genset derating * 1/0.9 charger eff * 0.10 rate of charge = 24,100 Watt genset minimum suggested (rough estimate)

I will stop here--I am sure you have lots of questions and would like to "correct" my guesses for a "better" answer.

-Bill

Wow, that is quite a massive response. I wasn't exactly expecting such a well-executed detailing of the ins and outs of what to expect. Such is worthy of RX7club moderator-hood. Well done mate.

I'm big on logical, rational efficiency and my mom was somewhat of an electric nazi, raising me to turn things off when not actively being used.

For simplicity, a 100 watt light bulb left on for one hour uses 100 watt-hours (0.1 kWh)

676 Square feet x 15 watts = 10140 watts generated per hour in full sun. 3 hours of this equals 30420 watts (30.42 KwH). Just above the national average usage per 24 hours in the typical suburban Susie Homemaker type house. I just used the national average as a baseline for worst case scenario usage. Based on the Insolation figures you provided, the only times this system wouldn't break even for Susie Homemaker on a daily basis are in December and January. Since I'm all about efficiency, my actual usage will be considerably less by design and behavior, meaning that I'd be further in the green and any extra would go into the battery bank. The end result will be somewhat of a house/cabin hybrid with a 40x40 mad scientist's garage...

I agree 110% that there's a LOT of "fat" built into the average house that eats power like candy. My goal is to eliminate 80% or more of it, while reusing a portion of the savings for more useful things. Water heaters are a big source of wasted watts, so going with tankless on-demand heaters is the answer. Assuming 4500 watts for 15 minutes to take a shower, it is 1.125 kWh used. And for the rest of the day, its power usage is virtually nothing. Quite a big savings compared to a conventional 40-gallon water heater (9.57 kWh per day). In layman's terms, tankless uses about 1/8 the power of a conventional water heater with the same power rating.

You are very correct in that getting measurements of actual usage is the bottom line here. Since I'm still in the planning phases, these ballpark figures are fine to work with for now.

14 years of RX7 ownership and being a moderator over there has taught me one lesson: The correct solution is never at the highest or lowest price point. I'll use a battery as an example. The loss-leader at Walmart MIGHT get you up and running, but it WILL leave you dead in the water and SOL. This is why I swear by Deka Intimidator AGM batteries. The one I had in my RX7 was their 9A48 battery, 70 AH capacity. I drained it to ~12.0v and recharged it at least 50 times, yet it kept coming back for more abuse. No idea why, I have no explanation. My friends and I named that battery Mjolnir.

In contrast, a similarly-priced AGM battery from Oreilly Auto Parts will fall on its face in 2 years and being discharged/recharged maybe 3 times, refusing to hold a charge for more than 2 days. They, of course, said the battery was "good" so no replacement under warranty. So I charge/discharge cycled it daily for a week with a 55w headlight bulb and then took it back. It magically tested "bad", and I got a new one right there for free. That battery was named Hamster, for how easily it died.

Lithium-Ion would be nice, but it sounds like a difficult and/or expensive option. Plus, there is merit to being able to visit any auto parts store and get an AGM battery at any time, instead of ordering online and waiting 2+ weeks for a specialized battery at higher cost.

Specialize where power savings is needed. Conventionalize where flexibility is needed. The solution to every issue is one of those two approaches.

SilverIris

Crap, I forgot to post the "why" for all of this. Summarized in one word: Kaizen. The simpler machine works better than the complicated one. It's also easier to understand and maintain yourself.

My dad was right about living frugally all along. So I'm using how he built his house (early 1970s, middle of nowhere in the woods, lucky to be able to get power and a landline at all) and taking it a step further. By removing the electric bill from the equation, it eliminates any residual effects such as random blackouts with zero explanation or easily discernible reason. That ****gets old quick when you're a professional translator, trying to read an ink smear scanned on a potato and what you write is how to service a system never before seen outside of Japan, and you're the only one in the world that can decipher it, based on knowing how the manufacturer "thinks". Now imagine that blackout happening twice in one day.

Anyway, remove the instability factors from your life and you become a happier person. Besides, if there ever was a cyberattack that breaks the power grid, I'll be "what was that noise?" while everyone is panicking.

mike95490

" plan is to have sufficient storage to cover 10 days of zero sun"

And this is not really feasible.  Day 3, the batteries start to sulfate, by day 10, a fair amount of damage will have been done.
 That for me, was a factor of choosing NiFe battery bank.  10-15 day stretches of cloud/rain is common here in winter. I
 use a small diesel generator as backup, and the same generator for you, could power the welder and spare the risk to the inverter.
Li batteries are an option, but wintertime, they may need some supplemental heat because they loose power when cool, and cannot be recharged below frost.  

Dave Angelini

People are all different, especially offgrid. It ranges from having back-ups to the back-up, to being fine with using candles and writing everything down with a pencil.

They could have used you on the manual in my Dad's (RIP) old 240Z he bought in the late 60's.

Good Luck on your journey!

BB.

Regarding battereis--There are a lot of choices out there... And while you are very happy with an AGM that discharged to 12.0 volts 50 times (something like 50% discharge), if you look at a good quality AGM battery designed for deep cycling, it should have a life of 1,000 Cycles (posted Lifeline battery manual here because it has lots of documentation and useful informaiton--Although the brand has excellent quality and reputation--and, like most AGM, are not cheap):

http://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101-Rev-E-Lifeline-Technical-Manual.pdf

And--If you were looking at 1,000 cycles--That is just over 3 years of cycle life--And then you are on the way to needing a new set of batteries.

Lead Acid deep cycle storage batteries are probably 1/2 the cost (roughly)... And Li Ion are much more than AGM. Although, Li Ion batteries have lots of other advantages that can make them cheaper to own overall (longer life, no chance of insulation, smaller bank AH capacity with higher output current and better charging characteristics).

And you are not going to be able to build a "reasonable" battery bank with a whole bunch of 12 volt @ 100 AH AGM batteries from the auto-parts store... For example, if you use my 2,941 AH @ 48 volt battery bank (which is still a fraction of what you seem to be planning).

Using Rolls 2 volt cells 
https://www.solar-electric.com/rolls-2-ys-27p-flooded-deep-cycle-battery.html

$952 each, need 24x in series @ 246 Lbs per cell (24x or 5,904 lbs).

To use 12 volt @ 100 AH, you would need 4x in series for a 48 volt @ 100 AH string * 22 parallel strings for 48 volts @ 2,200 AH or 88 total batteries...

https://www.solar-electric.com/lib/wind-sun/2-YS-27P.pdf (Rolls battery data sheet)

The Rolls FLA battery would last ~4,250 cycles to 50%  or 6,500 cycles to 75% SoC or 25% discharge... 6,500 daily cycles is ~17.8 years (at ~77F).

Would you get 18 years service life out of that cell??? I am not in the business, but probably 10-15 years with proper care and maintenance... Perhaps (I am not in the solar/battery business--You need to do some additional research from knowledgeable sources).

A good quality AGM battery (like Lifetime)--Is probably more of a ~7 year life battery. And a daily cycle "automotive" battery--Typical is 3-5 years in a car... Or even less if not deep cycle/storage battery designed (3-5 years for typical "golf cart" type batteries around 6 volt @ 200 AH per battery).

And when you are managing your own power company... Things like the the Rolls battery being 8 weeks out due to manufacturing lead time--Makes calling the electric company to get a power outage fixed in 1/2 a day with no money out of pocket seem like a pretty easy life.

If you have utility power--Getting a backup genset (plus possibly a small off grid solar system for lights, cell phone, laptop, RV water pump) and 5-10+ days of fuel is not a bad/lower cost solution...

I use the rules of thumb to quickly get to a "target drop zone" to see if this is even close to what you want (time, money, function)--And if it is not, then move on to another plan. As with anything like solar/gensets/etc., there is lots of room for optimization near the target... Trying to optimize a solution and having no idea where the target is within a 100 miles... It can get frustrating pretty quickly.

-Bill

SilverIris

These are all excellent points in regard to the variables one would encounter when going with an off-grid system for the first time. Chemistry is definitely NOT my strong point, so I am not the person to ask about storage battery chemistry. Also, a standard AGM battery discharged to 12.0v is approximately 75% discharged according to East Penn, the manufacture of Deka batteries. Pretty sure I ran it lower more than a few times, but it's been far too long to nail it down to an exact number. Either way, it was a good starting point that opened the doors to more suitable options.

Dave, funny you mention a 240Z (S30 chassis). I built my 20B RX7 using the "Akuma no Zetto" (Devil Z) as a benchmark. If you're ever looking for another S30, I've seen a few pop up for sale in Utah every so often.

BB, I've been doing some homework on inverters and came across another thread where you mentioned the fellow in Hawaii using AIMS inverters. Turns out they have a 30kW Split Phase inverter with appropriately beefy connections, seems like a decent starting point to benchmark from.
Link: https://www.aimscorp.net/30kw-pure-sine-power-inverter-charger-300-vdc-240-vac-split-phase.html

Granted, it's $12,000 but it seems like a correctly sized inverter is a "buy once, cry once" expense. Always better to have an overbuilt monster as the backbone of the power system than one that's as hearty as a hamster. Pretty sure most have seen the "hamster dies" memes, so you know exactly what I mean.

Here is the welder I have. Nothing giant, fancy or overkill, just a simple, inexpensive MIG for projects like stitching new rocker panels on my Expedition.
Link: https://www.ebay.com/itm/383828529248?_trkparms=amclksrc=ITM&aid=111001&algo=REC.SEED&ao=1&asc=20160908105057&meid=56a9a29e648847b4a007ace6833fdca4&pid=100675&rk=1&rkt=15&sd=383828529248&itm=383828529248&pmt=1&noa=1&pg=2380057&_trksid=p2380057.c100675.m4236&_trkparms=pageci:5ac71cff-3c19-11ec-9f67-2ad36cf9a83d|parentrq:e24917fa17c0a7b32ebb3471ffff195e|iid:1

Actually, having a dedicated inverter for Garage 220v devices might not be a bad idea. In case of a failure, it would be isolated from the "house" and have a far lower pricetag for correction due to easier availability. Plus, it would only have a few devices connected, and none of them would be operating simultaneously.

mike95490

That 30kw inverter weighs in at nearly 600 lbs  !    It also looks like it wants to have a 200-600v battery input. That's scary !
At +200VDC, I would start to worry about battery case breakdown voltage and other arcane problems.
   I'd study the shipping charges for return for repair too.

I think having several ganged 6.8kw inverters would give you redundancy and ability to change one out  ( 130lbs )  and the others will carry on

SilverIris

Agreed. There is a massive benefit to a 48v modular system vs a monolithic one at such high voltage. Plus, components have smaller pricetags, assuming its connection terminals are sufficiently up to par. Will have to fabricate two Junction Boxes to merge all of the ganged inverters' outputs together in the proper manner. Any suggestions on suitable inverters for such an application? I'm still a bit green on the subject, but unsynchronized parallel inverters sounds like bad juju waiting to happen.

BB.

There are AC inverters that can synchronize--Or even do 3 phase power (with 3 or more inverters).

-Bill

mike95490

The Schneider XW series.  Several can be "stacked" to meet the  load requirements.

And you will need Batteries and solar PV charge controllers.   To have that all wrapped up in one box, gives no redundancy.

Inverter ( stackable to 4 units )  https://solar.schneider-electric.com/product/xw-pro-na-solar-hybrid-inverter/

Solar Charge Controller ( 600V 80A )  https://solar.schneider-electric.com/product/conext-mppt-charge-controller-2/  Or the more conventional 60A 150V

master control panel  https://solar.schneider-electric.com/product/conext-gateway/

SilverIris

BB. said:

There are AC inverters that can synchronize--Or even do 3 phase power (with 3 or more inverters).

-Bill

My apologies Bill, it seems my research skills took a vacation the minute you mentioned 3-phase or synchronizing inverters. There seems to be quite a bit of misinformation and confusion regarding this. Could you please provide an example of the inverters you're referring to, so that I can see exactly the idea(s) you're referring to? Please keep in mind that I am quite green when it comes to such, so better safe than sorry.

Simply put, I have zero understanding of 3-phase power, despite my dad, a mechanical and electrical engineer, trying to explain it to me years ago. Granted, I was 8 at the time, but I think keeping it to the standard Split Phase 220v and scaling capacity to suit would be the simplest solution to such a complex equation.

mike95490

unless you have electric motors over 1hp, or large shop machines, 3 phase power is overkill. Instead of 2 phases like a split phase inverter produces, you have 3 phases,  120 deg out of phase with each other ( instead of 2 phase being 180deg out)
https://en.wikipedia.org/wiki/Three-phase_electric_power

BB.

Here is one (higher end) 3-phase capable inverter:

https://www.solar-electric.com/conext-xw-8548-61-230-volt-inverter-48v-charger.html

I am not suggesting it is a solution for you... If you had motors that were >1 HP, then three phase can make sense...

Just to give you the basics of three phase power--Here is an explanation of how a 3 phase alternator works (3-phase AC inverter set--Looks just like a 3 phase alternator output):

https://electrical-engineering-portal.com/few-words-about-three-phase-alternator

3 phase has some nice features--One is it is a "naturally" rotating field for motors (don't need starting capacitor, etc. to get motor to run). Also 3 phase power is constant power (very smooth motor turning). Single Phase power "pulses" at 120 per second (60 Hz +/- sine wave).

However, it tends to be more expensive (3x inverters, 3x transformers, 3 wire AC and 3 breakers/switches vs 1 or 2 "hot wires" for 120 or 240 VAC power).

In North American homes, very few (if any) are going to have three phase power (unless you are on a farm or something).

And 3 phase power systems can be very complex to setup for off grid power... Here is an example of a (high end off grid system in a country that does have 3 phase power to homes) system that turned out the first Power Engineer designed and installed, but could not fix the problems make the system run reliably:

https://forum.solar-electric.com/discussion/17986/is-the-system-set-up-right/p1

Defining your loads (peak/average Watts, type of power 120/240 VAC, Split phase or not, poly phase?, etc.) and your daily kWH needs. Then a paper design of the basic system. Then finally, start looking for hardware that will support those needs. Lastly, start buying hardware once you have everything defined and configured.

The loads will suggest what hardware you will need... I.e. are you looking for a compact car, pickup, or 18 wheel tractor/trailer... The size of the loads will dictate what hardware will work for your needs (and what options are available for hardware).

-Bill

SilverIris

mike95490 said:

The Schneider XW series.  Several can be "stacked" to meet the  load requirements.

And you will need Batteries and solar PV charge controllers.   To have that all wrapped up in one box, gives no redundancy.

Inverter ( stackable to 4 units )  https://solar.schneider-electric.com/product/xw-pro-na-solar-hybrid-inverter/

Solar Charge Controller ( 600V 80A )  https://solar.schneider-electric.com/product/conext-mppt-charge-controller-2/  Or the more conventional 60A 150V

master control panel  https://solar.schneider-electric.com/product/conext-gateway/

So I read through the datasheets and the XW Pro 6848 NA sounds like a good benchmark to compare to. 6.8kW at anything under 77 degrees Fahrenheit, 6.0kW at 104 degrees, and 8.5kW/30 minutes and 12kW/60 seconds Surge capacity. 4 of them together is roughly 80% of the average "karen" electric consumption, so I can definitely make it work. Based on previous electric bills, I could probably get away with 15-20kW, but it never hurts to play it safe and have more capability than one needs.

SilverIris

So I did some digging and this came up:
https://www.signaturesolar.us/products/5kw-48w-240vac-80a-off-grid-inverter-by-growatt

Manufacturer Link: https://www.ginverter.com/Off-Grid-Storage-Inverters/44-625.html

5.0kW sustained load, 10kW Surge capacity, stackable up to 6 units via Canbus, 26 pounds each, and a pricetag of $899 each on Ebay. Pretty sure I could find a lower price somewhere, but $5400 for "karen" capacity is hard to beat.

The only downside is that it requires a Split Phase transformer to output standard US 220v. Not sure how that works (yet), but I think we may have a winner here.

mike95490

Price out a 15KVA transformer w/shipping.   Transformers are "nice" but bring 2 issues to the table:
a) Power Factor and it's own inductive load on the inverter
b) Surge capacity.  The iron core can only store so much magnetic field and then it collapses when overloaded. So the transformer has to be rated at your surge requirement, which can be a large transformer.

if you know what you are looking for, flea bay has some good bargins, i got several used ones w/ free shipping

SilverIris

mike95490 said:

Price out a 15KVA transformer w/shipping.   Transformers are "nice" but bring 2 issues to the table:
a) Power Factor and it's own inductive load on the inverter
b) Surge capacity.  The iron core can only store so much magnetic field and then it collapses when overloaded. So the transformer has to be rated at your surge requirement, which can be a large transformer.

if you know what you are looking for, flea bay has some good bargins, i got several used ones w/ free shipping

Excellent advice and greatly appreciated. So I did some hunting and came across this:
https://store.maddoxtransformer.com/products/1-phase-240x480-120-240?variant=12738823225427&utm_term=&utm_campaign=Smart+Shopping+-+SG&utm_source=adwords&utm_medium=ppc&hsa_acc=8133699481&hsa_cam=1748409503&hsa_grp=75307175744&hsa_ad=340485292318&hsa_src=u&hsa_tgt=pla-325425753764&hsa_kw=&hsa_mt=&hsa_net=adwords&hsa_ver=3&gclid=Cj0KCQjw5oiMBhDtARIsAJi0qk0Ptr6dLNCgj4trqgQDDl7DvQyXU9nGvtC4S70ZFcWJEX6xlC2cJPoaAqM8EALw_wcB

37.5 KVA, input is 240v or 480v single phase to 120/240v split phase output, and plenty of "overhead" capacity for my purposes. A bit "industrial" for some people, but that's fine by me. I've found that when doing what everyone says is "impossible", thinking outside the box always helps. New is $1670, Used is $1169. Shipping would be ~$200 based on their claim of 5-10% of item price.

They also have 15KVA ones for $820 new, $574 used. What do you think? I just realized that if it's placed it in the garage area, any heat it produces is put to good use to keep the garage warm in the winter. Sounds like a pretty clever bonus to me

Edit:
50KVA transformer is $1481 new. Shipping is probably about the same as the 37.5KVA one, but more capacity and cheaper. Win Win.
https://store.maddoxtransformer.com/products/1-phase-240x480-120-240?variant=12738823880787&utm_term=&utm_campaign=Smart+Shopping+-+SG&utm_source=adwords&utm_medium=ppc&hsa_acc=8133699481&hsa_cam=1748409503&hsa_grp=75307175744&hsa_ad=340485292318&hsa_src=u&hsa_tgt=pla-325425753764&hsa_kw=&hsa_mt=&hsa_net=adwords&hsa_ver=3&gclid=Cj0KCQjw5oiMBhDtARIsAJi0qk0Ptr6dLNCgj4trqgQDDl7DvQyXU9nGvtC4S70ZFcWJEX6xlC2cJPoaAqM8EALw_wcB

mike95490

Nice transformers, but you need one with a center tap winding, to create 120/240Vac, an AutoTransformer
  Also, will your chosen inverters handle that much of an inductive load ?
Transformers have about 5% loss. Going much larger then the expected surge capacity, is just wasting power keeping the core warm.

Here's a link to a large AutoTransformer.   See how they have a shared common winding
https://canadatransformers.com/37-5-kva-autotransformer-mc37c-a.html
  However, this one is labeled 240  to 120V.  It should work ok for 120V to 240v split phase..  Check with engineering first.

Dave Angelini

Alot of the Chinese design inverters use peak power in their specifications. XWP is RMS power backed by a company that will be there down the road.

mike95490

5.0kW sustained load, 10kW Surge capacity, stackable up to 6 units via Canbus, 26 pounds each, and a pricetag of $899 each on Ebay. Pretty sure I could find a lower price somewhere, but $5400 for "karen" capacity is hard to beat.

Vs 6.8kw  120lbs, stackable

SilverIris

Mike, I have a question about your reply in regard to my idea for 10 days of storage capacity, quoted below:
"And this is not really feasible.  Day 3, the batteries start to sulfate, by day 10, a fair amount of damage will have been done.
 That for me, was a factor of choosing NiFe battery bank.  10-15 day stretches of cloud/rain is common here in winter. I
 use a small diesel generator as backup, and the same generator for you, could power the welder and spare the risk to the inverter.
Li batteries are an option, but wintertime, they may need some supplemental heat because they loose power when cool, and cannot be recharged below frost.   "

Since the solar array would be sized to fulfill 30kWh in 3 hours, and anything beyond that being fed to the battery bank to charge it and/or maintain full charge, one would imagine that the battery bank would be almost always at or near full charge in regular daily operation. That is, barring freak circumstances to create extended no-sun conditions such as a snowstorm akin to the Blizzard of 1978. Also, as my actual usage would be less than the 30kWh figure I referenced, the battery bank capacity would be under less load, stretching more time out of every AH available.

I have not designed the power room yet, but based on all of the factors presented, it seems like having it as part of the heated "house" section would make logical sense.

From my understanding, Sulphation occurs due to allowing a battery to remain discharged, overcharged or excessive storage temperatures, reducing the surface area of the internal lead plates, and reducing its capacity to store electrical energy as a result. So based on all of this, is there a factor that I am missing here that would result in Sulphation in my intended backup system? Could you please clarify what you mean.

BB.

Just to be clear here... You are asking about Flooded Cell Lead Acid deep cycle batteries and NOT Li Ion or other types?

FLA (and AGMs that are getting old and/or over charged) do "gas" hydrogen and oxygen. As well as some sulfuric acid (electrolyte) mist...

You wan the battery room (or battery box) vented to prevent Hydrogen gas build up--And to prevent electrolyte "mist" from damaging items/electronics/wiring in the area (and some folks are very sensitive to the "rotten egg" smell of sulfuric acid).

FLA and AGM batteries "like" and do well in 77F to near 32F. And FLA/AGM batteries last much longer even down below 0F (aging life). Yes, they become less chemically "active" (lower apparent capacity) when cold--But this is only when cold, causes no damage, and will recover when warm again.

Li Ion batteries--Different animal. They do not charge at roughly 40F or less. They do not vent in normal operation--But if they ever "catch fire"--They can spew some very dangerous chemicals which can make your place a super fund waste site (a bit of an exaggeration, but hydrofluoric acid is highly toxic and can cause permanent injury or death with even limited exposure and typically cannot be "cleaned"--Contaminated materials must be dumped).

People run battery banks in homes/basements all the time--But I do suggest that you vent well, build to code, and do not use "wood" construction around batteries/electrical wiring. But use some sort of metal boxes/conduit/non-flammable materials instead.

FLA (and to some extent AGM), batteries "sulfate" when stored at less than roughly 75% State of charge. The conversion of "fluffy gray" lead sulfate to hard black Lead Sulfate crystals which take lead/sulfur permanently out of the charge/discharge process. And the sulfation starts in hours and can kill the batteries in months (stored non-cycling under 75% SoC).

An "actively" cycling FLA battery... Suflation does not occur. In fact, there is daily use profile of (for example) discharging to 50% to 80% SoC daily, and only recharging >90% once a week or once a month. But this is different than discharging down to 50% and letting bank sit for days/weeks with no recharging. This type of cycling is just not for FLA (and AGM) batteries.

Li Ion batteries (and other chemistries) are not damaged by sitting at partial state of charge (most Li Ion types even last longer if stored at 40% SoC). But Li Ion batteries are not cheap and usually need a BMS (battery management system) for proper operaition.

-Bill

mike95490

When you asked about 10 days of zero sun

" plan is to have sufficient storage to cover 10 days of zero sun"

your 30kw array might be producing 500w.   That means your inverter is sucking power from the battery for 10 days, and the battery not getting recharged.   As power is removed from LA batteries, the acidic electrolyte deposits sulfur on the battery plates and the acid weakens.  The longer the sulfur deposits sit, the harder and more firmly bonded to the plates it ( sulfur ) becomes.  Prompt recharging
drives the sulfur back into the acid and the battery is all well.   But after a day or 2, the bonding becomes so strong it is difficult to drive the sulfur back into solution and so you loose battery capacity.   Big trouble after 5 days, near total loss of bank at 10 days.

Plain cloudy days decrease harvest, it doesn't have to be a storm  with your neighbors lawn chairs blown through your roof top panels !

SilverIris

Thank you for clarifying, I was not aware of how quickly permanent sulphation can occur once the process begins. Thankfully, a little due diligence can mitigate such a problem. Actually, this gives me an idea. Anyone ever thought of logging their Charge Controller(s) data to a SD card, referenced to a Real-time clock? In the car modding community, aftermarket ECUs such as the Megasquirt MS3-Pro can log every engine parameter (Manifold pressure, battery voltage, RPM, injector duty cycle, etc), with user-defined "alarm points" to a file saved to an onboard SD card. With that file, you can play back what it recorded through the MS3-Pro software, just like a VHS cassette (Yes, I'm that old... ). All it takes is a battery inside the MS3-Pro to operate the clock.
The same thing could be done here with an Arduino and SD card shield (board). If you want to get fancy, add a Wifi or Bluetooth shield so it can send a message when things go out of whack in the manner you've suggested.

Here's a little preview of what I have in mind, drawn up in Sweet Home 3D

First Floor:



Second Floor:

mike95490

A simple logging voltmeter could provide a record of battery voltages. 

Nearly every charge controller mfg, has a different data language, so you may have to "roll your own" decoder to
capture things from the data stream.

BB.

Modern Integrated charge controllers/AC inverter/and even some BMS equipped Li Ion batteries do that--And are even Internet connected (data, configuration, alarms).

Another option is that some charge controllers (Midnite with their Wizbang Jr. remote battery current sense and possibly Outback with their "Flexnet" system).

As you get more complex--There is more to configure, more functions, and possibly a bit more to debug if/when things go wrong.

-Bill