# What does "48-volt inverter" mean?

Registered Users Posts: 29 ✭✭
I am starting to get serious about designing my solar-electric system in detail.

My charge controller, inverter and battery bank will be located in the basement of my house.

The cable run from my PV array to my charge controller, inverter and battery bank will be about 150 feet. To minimize voltage drop, I think I need to push 48 volts (or more) from the PV array to the charge controller, and I think I need to use at least 8AWG cabling.

My main question is "When somebody uses the term "48-volt inverter" in the context of discussion about a solar-electric system, do they mean that the feed coming from the PV array to the charge controller is a 48-volt feed?

There are lots of things I need to work out, but it would help a lot if I can just understand this basic concept.

• Solar Expert Posts: 6,006 ✭✭✭✭✭
Inverters work off the system voltage. System voltage is defined by the battery bank. If you have a 12 volt bank you would need a 12 volt inverter. 24 volt battery bank...
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Solar Expert Posts: 2,084 ✭✭✭✭✭
With a MPPT charge controller you want your incoming voltage to be 1 1/2 to 2 times the nominal battery bank voltage. You can go higher especially in case of long wire runs from the array to the controller. MPPT controllers typically  have a 100 to 150 volt limit. Some go as high as 600 volts.
With a PWM controller you bring in 48 volt nominal voltage. Which would be 4, 12 volt panels in series or 2, 24 volt panels in series for example. Actual voltage being roughly 68 to 74 volts.

A 48 volt inverter wants a 48 volt battery bank period regardless what the charging source ie: solar, wind, hydro, generator etc.

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

• Solar Expert Posts: 3,854 ✭✭✭✭✭✭
I am starting to get serious about designing my solar-electric system in detail.

My charge controller, inverter and battery bank will be located in the basement of my house.

The cable run from my PV array to my charge controller, inverter and battery bank will be about 150 feet. To minimize voltage drop, I think I need to push 48 volts (or more) from the PV array to the charge controller, and I think I need to use at least 8AWG cabling.

My main question is "When somebody uses the term "48-volt inverter" in the context of discussion about a solar-electric system, do they mean that the feed coming from the PV array to the charge controller is a 48-volt feed?

There are lots of things I need to work out, but it would help a lot if I can just understand this basic concept.

The reference to 48 volt is the DC input voltage of the inverter, typically they come in 12, 24 and 48V, so depending on the battery bank voltage, the inverter voltage would match the battery nominal voltage .The higher the DC input voltage  to the inverter, the less the current for a given output load, this allows for smaller conductors, fusing/circuit breakers etc. on the DC side. The PV voltage is determined by the maximum input of the charge controller, this can be significantly higher than the battery/inverter  voltage, the controller will reduce the PV voltage to match the requirements needed to charge the bank.

NOTE. Only MPPT charge controllers can use higher PV input voltages

Using a higher DC voltage has benifits, especially for higher output loads and allows for less parallel strings in battery configuration and the aforementioned ballance of system benefits. The determining factors for choosing which voltage to use would be based on load demand but as a general rule it is better to use a higher voltage when loads approach or exceed 2000 watts.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
• Solar Expert Posts: 9,583 ✭✭✭✭✭
What are your expected loads and WH of your battery bank ?  That makes it easier to decide the voltage of the system and then the rest follows
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users Posts: 29 ✭✭
mike95490 said:
What are your expected loads and WH of your battery bank ?  That makes it easier to decide the voltage of the system and then the rest follows
My expected daily usage is 2.5 KWH.

I probably won't need to pull more than 1500 watts at a time.

The battery bank I have in mind will provide 38,400 WH of storage. It will consist of 16 Trojan J200-RE 12V 200AH batteries. Not sure if I should do 2 strings of 8 batteries or 4 strings of 4 batteries.

FYI, I live pretty far north. According to a couple on-line sources, average daily sun-hours in my location is 2.5.
• Solar Expert Posts: 6,006 ✭✭✭✭✭
My expected daily usage is 2.5 KWH.

I probably won't need to pull more than 1500 watts at a time.

The battery bank I have in mind will provide 38,400 WH of storage. It will consist of 16 Trojan J200-RE 12V 200AH batteries. Not sure if I should do 2 strings of 8 batteries or 4 strings of 4 batteries.

FYI, I live pretty far north. According to a couple on-line sources, average daily sun-hours in my location is 2.5.
That's a pretty huge battery bank, I suspect with those minimal hours of sunlight you will have alternate means of charging?

I would think to be cost effective you would want a smaller battery bank, more in the 15-20Kwh range, and charge from the alternate source if needed. Even then many would think it over sized.

With an average of 2.5 hours of sun, I hope that is during the winter months and not a year round average?

For daily use we like to use a 10% charging rate, for a 38Kwh battery bank that would be an array capable of 7,000 watts an hour. even if we figure it will be very cold during the minimal hours of sun light that's a 7,000/7500 watt array...

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Registered Users Posts: 4,496 ✭✭✭✭✭
With a big bank (800ah@48v) you may want to consider large 2v cells rather than lots of 12v. With 12v you will have 12x6=72 cells to check SG and water vs 24@2v. Also, 12v requires 4 parallel strings of batteries which can be tricky to keep balanced.

Depending how far north you are, you may want to throw out the worst 2 or 3 months for your average.

At 150' using strings of ~30vmp x 3 for 90-100v with 8 gauge should work.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Registered Users Posts: 29 ✭✭
Photowhit said:
My expected daily usage is 2.5 KWH.

I probably won't need to pull more than 1500 watts at a time.

The battery bank I have in mind will provide 38,400 WH of storage. It will consist of 16 Trojan J200-RE 12V 200AH batteries. Not sure if I should do 2 strings of 8 batteries or 4 strings of 4 batteries.

FYI, I live pretty far north. According to a couple on-line sources, average daily sun-hours in my location is 2.5.
That's a pretty huge battery bank, I suspect with those minimal hours of sunlight you will have alternate means of charging?

I would think to be cost effective you would want a smaller battery bank, more in the 15-20Kwh range, and charge from the alternate source if needed. Even then many would think it over sized.

With an average of 2.5 hours of sun, I hope that is during the winter months and not a year round average?

For daily use we like to use a 10% charging rate, for a 38Kwh battery bank that would be an array capable of 7,000 watts an hour. even if we figure it will be very cold during the minimal hours of sun light that's a 7,000/7500 watt array...

I just did a couple more lookups for average daily sun hours in my area, and from these new sources I'm getting 4-5 hours. This is an average over a 12-month period,

Here is the math I did to confirm (I think) that the size of the battery bank is appropriate for my situation. Please critique.

As I said, the battery bank I have in mind will provide a nominal 38,400WH of storage. (12V 200AH for each battery = 2400WH. 16 batteries x 2400WH = 38,400WH.)

It's my understanding that the internal resistance of many FLA batteries is 15%. If I assume  the 15% factor is anywhere near accurate for the Trojan J200-RE 12V battery, then I'll actually have only 32,640WH available in the battery bank.

To promote long battery life, I don't want to draw the batteries down below 70% SOC, which means I don't want to use more than 30% of the 32,640 available WH. This leaves me with 9792WH,

If I pull 2.5 KWH per day out of the battery bank, I'll have just under 5 days of autonomy. (I'd rather have at least 10 days of autonomy, but I'll settle for 5.)

Does this make sense?

RE <<I would think to be cost effective you would want a smaller battery bank, more in the 15-20Kwh range, and charge from the alternate source if needed. Even then many would think it over sized.>>

I do have a generator, and I would use the generator as needed to maintain a >= 70% SOC. However, I prefer to use the generator as little as possible. I absolutely want to be over-paneled so I won't need to use the generator much. Based on your figures, if I went with 300W panels I'd need 24 of them.

• Solar Expert Posts: 6,006 ✭✭✭✭✭
MyPrepperLife said:

I just did a couple more lookups for average daily sun hours in my area, and from these new sources I'm getting 4-5 hours. This is an average over a 12-month period,

It's my understanding that the internal resistance of many FLA batteries is 15%.

To promote long battery life, I don't want to draw the batteries down below 70% SOC, which means I don't want to use more than 30% of the 32,640 available WH. This leaves me with 9792WH,

If I pull 2.5 KWH per day out of the battery bank, I'll have just under 5 days of autonomy. (I'd rather have at least 10 days of autonomy, but I'll settle for 5.)

Does this make sense?

RE <<I would think to be cost effective you would want a smaller battery bank, more in the 15-20Kwh range, and charge from the alternate source if needed. Even then many would think it over sized.>>

I do have a generator, and I would use the generator as needed to maintain a >= 70% SOC. However, I prefer to use the generator as little as possible. I absolutely want to be over-paneled so I won't need to use the generator much. Based on your figures, if I went with 300W panels I'd need 24 of them.

"internal resistance" is important in the flow of energy to charge the battery, this is the amount you need the incoming voltage to be greater than the resting voltage to charge the battery. It should not be worried about in battery capacity IMHO.

To build a system it's the months of minimum solar energy that you need to plan for, when properly designed for these the days with greater sunshine will take care of them selves. (Unless your loads change, I run an Air conditioner in the summer time so have dramatically higher loads) Though if the system is so over built you might want to take the batteries SOC level down a bit when it has had a couple weeks without dropping 5%. The dirty secret about lead acid batteries is they like to be used!

I understand this is a "prepping" model, but long battery life is also added by having a bank with many strings of batteries would also effect battery life, as it's hard to maintain equal resistance across each string.

A more reasonable sized system would allow for a good deal of stored propane to run a generator, Propane doesn't go bad, but might need to be buried or other wise protected from very cold temps.

What I sized is a minimum, 13% of stored amperage would be on the high end and 15% would begin over paneling!

I'm a bit over paneled now with 5Kw of array feeding my (supposed) 800 Ah 24 volt battery, in a warmer climate (panel rating is about 75% of actual output of hot panels. SEE NOCT values.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Registered Users Posts: 4,496 ✭✭✭✭✭
Battery capacity is typically quoted at a 20hr discharge rate. The apparent capacity will be lower at higher discharge rates, and higher at lower discharge rates. Aiming for 5 days to discharge to 70% is a much slower rate so apparent capacity would be somewhat higher than the 200ah quoted, not 15% less.

Apparent capacity is also affected by temperature. The chemical reaction slows at lower temps, which lowers apparent capacity. If you plan to have the bank in a space that gets cold (eg. <0C) this could be significant.

In most circumstances we would consider your bank to be oversized for the job. Properly maintained it will die of old age before it "wears out" from use. Unless fuel costs are really high, the life cycle cost would be lower with a smaller bank and a bit more generator use. That said, loads seem to grow over time.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Solar Expert Posts: 6,006 ✭✭✭✭✭
Estragon said:
In most circumstances we would consider your bank to be oversized for the job. Properly maintained it will die of old age before it "wears out" from use. Unless fuel costs are really high, the life cycle cost would be lower with a smaller bank and a bit more generator use. That said, loads seem to grow over time.
This is very important and might be something you don't understand or have over looked.

Batteries have a finite life, even if a company tells you it will last 2000 cycles. The timer starts ticking when they are made. The thicker the lead plates, typically the longer they last.

For lead antimony (most US deep cycle FLA batteries) figure a max life of 7 years or so for golf cart size batteries, 12 years for L-16 sized batteries and 20 years for forklift batteries. We've all heard of longer lasting batteries, but these would be realistic maximums with extra years considered a bonus. For planning proposes, 5 - 7 - 10 would be more realistic. They will also loose some capacity as they age.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Registered Users Posts: 29 ✭✭
edited February 2017 #13
Yeah, Photowhit, I wasn't sure about my use of an internal-resistance factor in my calculations.

There is quite a bit of conflicting info on the Web concerning internal resistance, derating factor, etc, for a battery bank. I think I just won't concern myself with it, at least for now, because - according to what I've learned from this thread - shooting for a minimum 70% SOC is a pretty conservative way to go, so if there is some "derating" going on, my battery bank will have enough of a cushion to handle it. I guess?

The more I learn about solar-electric technology, the more I realize how much I still don't know! (As is the case with so many things in life...)

Anyway, maybe I should step down the size of the battery bank.

Estragon, thanks for your suggestion about using 2V batteries instead of 12V batteries so I'd have fewer cells to check. Based on your suggestion, I just took a look at this 2V 1100AH Trojan battery, and it seems like a likely candidate. 12 of these would create a 26,640WH battery bank. At a minimum 70% SOC, this would give me 7992WH, which would be 3-4 days of autonomy.

BTW, any time I mention capacity (AH) for a particular battery, I'm using the 20-hour figure given in the specs for that battery.

Any guesses as to how much panel wattage I would need to keep a 26,640WH battery bank full most of the time?

My opposition to charging the battery bank with my generator isn't based solely on an aversion to purchasing fuel for it; it's also based on a couple other things: 1. Aversion to the inconvenience of dealing with the generator more frequently than I would like - refueling it - changing the oil - etc. 2. I want to plan for circumstances when perhaps it would be difficult or impossible to get fuel, because, as you can tell from my username, I'm a prepper. A survivalist, even.

BTW, one other thing I want to mention (I don't think I've mentioned it already in this thread) is that my battery bank will be located in an insulated but unheated area of my basement. In winter, the temperature in that area remains at about 40 degrees F most of the time. Probably the batteries would be happier in a warmer location, but that's where they will be because that's the place I have reserved for them.

• Solar Expert Posts: 6,006 ✭✭✭✭✭
edited February 2017 #14
The battery location is fine, you will likely want to vent the area to the outside, some hydrogen will be released as batteries charge and discharge.

The battery you have linked to is somewhat suspect from many of us. It's actually 3 - 2 volt cells linked in parallel. Trojan, as well as many others, Crown, Rolls Surette, make 2 volt cells that are just 1 cell. I would prefer a single cell.

How large an array will vary by usage, and local solar isolation. In the sunny southeastern US a 5% charging rate for some one with minor usage might be fine. Trojan recommends a 10-13% charge rate, a figure we kick around for systems in daily use. An 1100 amp hour 24 volt battery, would want 110 amps charging. if the array is cold during your high demand time, no derating for hot cells/panels, that would be around (110amps x 29 volts=) 3190 watts for a minimum about 10% charge rate and (3190x1.3=)4147 watts for a 13% charge rate.

A 48 volt system voltage might offer some advantages, you won't be running very high wattage so the wiring won't be a huge expense, but at 48 volts a single charge controller might be all you need up to @5000 watts, while a 24 volt system will need 2 charge controllers. At \$500-1000 a significate expense.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Registered Users Posts: 29 ✭✭
edited February 2017 #15
Photowhit said:
The battery location is fine, you will likely want to vent the area to the outside, some hydrogen will be released as batteries charge and discharge.

The battery you have linked to is somewhat suspect from many of us. It's actually 3 - 2 volt cells linked in parallel. Trojan, as well as many others, Crown, Rolls Surette, make 2 volt cells that are just 1 cell. I would prefer a single cell.

How large an array will vary by usage, and local solar isolation. In the sunny southeastern US a 5% charging rate for some one with minor usage might be fine. Trojan recommends a 10-13% charge rate, a figure we kick around for systems in daily use. An 1100 amp hour 24 volt battery, would want 110 amps charging. if the array is cold during your high demand time, no derating for hot cells/panels, that would be around (110amps x 29 volts=) 3190 watts for a minimum about 10% charge rate and (3190x1.3=)4147 watts for a 13% charge rate.

A 48 volt system voltage might offer some advantages, you won't be running very high wattage so the wiring won't be a huge expense, but at 48 volts a single charge controller might be all you need up to @5000 watts, while a 24 volt system will need 2 charge controllers. At \$500-1000 a significate expense.
Photowhit said:
The battery location is fine, you will likely want to vent the area to the outside, some hydrogen will be released as batteries charge and discharge.
The battery you have linked to is somewhat suspect from many of us. It's actually 3 - 2 volt cells linked in parallel. Trojan, as well as many others, Crown, Rolls Surette, make 2 volt cells that are just 1 cell. I would prefer a single cell.

How large an array will vary by usage, and local solar isolation. In the sunny southeastern US a 5% charging rate for some one with minor usage might be fine. Trojan recommends a 10-13% charge rate, a figure we kick around for systems in daily use. An 1100 amp hour 24 volt battery, would want 110 amps charging. if the array is cold during your high demand time, no derating for hot cells/panels, that would be around (110amps x 29 volts=) 3190 watts for a minimum about 10% charge rate and (3190x1.3=)4147 watts for a 13% charge rate.
A 48 volt system voltage might offer some advantages, you won't be running very high wattage so the wiring won't be a huge expense, but at 48 volts a single charge controller might be all you need up to @5000 watts, while a 24 volt system will need 2 charge controllers. At \$500-1000 a significate expense.

Thanks for steering me away from the Trojan L16RE. While in this preliminary planning stage I will instead focus on the Trojan battery you depicted in the image embedded in your post: the Trojan IND27-2V,

Of course, if we're talking about 2V batteries, the best I could do would be a 24V battery bank unless I want more than 12 of them, which I don't. If my understanding of the math is accurate, what I'd have is just one string - right? I guess having only one string would be a good thing, as people on various forums, including this one, keep saying it's difficult to keep multiple strings of batteries "in balance." Whatever that means.

If you will continue to indulge me, I'd like now to ask if you think a 24-volt battery bank would meet my needs.

My battery bank will need to power the following (not all at the same time):

- 900-watt microwave (for just a few minutes per day)
- 1000-watt hair dryer (for just a few minutes 2-3 times per week)
- office equipment: laptop, tablet, inkjet printer, signal booster for cell phone, modem, router
- small electric clothes washer and propane clothes dryer (3-4 wash-and-dry cycles per month - would do laundry only when sunny)
- lighting (I use energy-efficient light bulbs. Probably won't use more than 100 watts per day for lighting, if that)
- submersible well pump

All of this adds up to < 2KWH per day. I'm using 2.5KWH per day in the calculations for my solar-electric system; the extra .5KWH is just a "cushion."

Notice that I haven't mentioned refrigeration appliances or cooking appliances, nor have I mentioned a water heater, nor have I mentioned a TV or radio or CD player, nor have I mentioned a vacuum cleaner, nor have I mentioned power tools. Here's why:

My refrigerator and freezer will be powered by a separate 12-volt solar-electric system. That system has been up and running since last November.

My cooking and baking appliances are fueled with propane. In addition, I'll have a woodstove with an oven; I can (supposedly) use this for cooking and baking.

My water heater will be a Bosch propane-fueled on-demand tankless water heater than uses just a tiny amount of electricity.

I don't have and don't want a TV. I use my tablet to stream movies, series, etc. from Netflix and Amazon.

I have a battery-operated combo radio and CD player.

I have a small, hand-held vacuum cleaner that meets a lot of my needs. If I need to operate a powerful vacuum cleaner once in awhile, I use my generator.

If I need to use power tools that need AC power, I'll use my generator.

Regarding your comments about charging rates: Wouldn't my charge controller handle this automatically and correctly? After I configure it properly, I mean.

To expand on the question a bit: there is one aspect of the charging scenario that I've been wondering about lately. Let's say there is a scenario where my panels aren't producing much electricity. For example, maybe they are still in shadow because the sun needs to move a little before they get full sun (this will happen every morning at my house). Or maybe it's a really cloudy day. Will a good MPPT charge controller be smart enough to refuse to charge the batteries if the juice it's receiving from my PV array is insufficient to charge the batteries at the recommended charge rate? I would hope so.

• Registered Users Posts: 29 ✭✭
edited February 2017 #16

I'm pretty sure I'm now understanding why folks here are telling me I don't need the huge battery bank i thought I needed.

FIRST: Since I am designing the system for multiple days of autonomy, I should of course be using the 72-hour capacity (AH) rating or even the 100-hour capacity (AH) rating in my calculations - not the 20-hour capacity (AH) rating. Using the 72-hour or 100-hour rating as opposed to the 20-hour rating makes a huge difference.

SECOND: Reading between the lines, it seems to me some people here think I could and should go lower than the 70% minimum SOC I thought I should maintain. Is a 50% minimum SOC a better choice? I'm feeling pretty good about 50% because the Trojan Battery Web site states that their industrial-line batteries are "designed for 2800 cycles at 50% DOD" - and, based on input from Photowhit, I am now considering one of the Trojan industrial batteries - the Trojan IND27-2V.

So let me take another stab at this, assuming I'll use the Trojan IND27-2V battery.

The data sheet for that battery gives a 100-hour capacity (AH) rating of 1954, assuming a cutoff voltage of 1.75 vpc. (I don't yet have a good understanding of cutoff voltage; I am just taking this information from the data sheet.)

This battery is a 2-volt battery, so the capacity of each battery in WH is 2 x 1954AH, or 3908WH.

If I wire 6 of these batteries in series, I'll have a battery bank with a capacity of 23,448WH (6 x 3908WH).

11,724WH will be available to me if I draw the batteries down to a 50% SOC over a period of several days. (11,724WH is 50% of 23,448WH.)

Assuming usage of 2.5KWH (2500WH) per day, I'll have almost 5 days of autonomy (11,724WH available / 2500WH per day = 4.6896 days). I could live with that.

Now, the only potential problem I can discern is that the 6 2-volt batteries wired in series will result in a 12-volt battery bank rather than a 48-volt or 24-volt battery bank, which is what we've been discussing in this thread till now. And I don't even know if this is actually a problem. Is it?

Somebody please tell me if I am on the right track!
• Solar Expert Posts: 6,006 ✭✭✭✭✭
I have a small, hand-held vacuum cleaner that meets a lot of my needs. If I need to operate a powerful vacuum cleaner once in awhile, I use my generator.

If I need to use power tools that need AC power, I'll use my generator.

Regarding your comments about charging rates: Wouldn't my charge controller handle this automatically and correctly? After I configure it properly, I mean.

To expand on the question a bit: there is one aspect of the charging scenario that I've been wondering about lately. Let's say there is a scenario where my panels aren't producing much electricity. For example, maybe they are still in shadow because the sun needs to move a little before they get full sun (this will happen every morning at my house). Or maybe it's a really cloudy day. Will a good MPPT charge controller be smart enough to refuse to charge the batteries if the juice it's receiving from my PV array is insufficient to charge the batteries at the recommended charge rate? I would hope so.

I think, as described it will meet your needs, we still don't know what your minimum average month is, but if you are willing to use the genny on those periods on cloudy days, you should be fine.

This is a good sized system. Mostly I would just load shift and use power tools and vacuums and such on sunny afternoons when the batteries are topped off. I do and don't own a generator.

No, the charge controller will bring the batteries up slowly and charge with what power is available. You may want to include a monthly equalizing, particularly with tall case batteries to mix the electrolyte and somewhat equalize the cells. I prefer to divide equalizing into 2 groups a maintenance equalizing and a corrective equalizing.

You will not discharge through the charge controller, but if there is enough energy to raise the voltage to charging levels, the charge controller will start charging.

The charging cycle is generally in 3 stages;

Bulk, when the charge controller passes as much energy through to the battery as it will take.

Absorb, when the battery reaches about 80% full the voltage will run away without the charge controller, the charge controller holds the voltage at a set level. at this point the battery slowly reduces the amount of current that it will accept. This can be a timed setup or on most modern charge controllers, set to use the amount of amps passing through to end the absorb cycle.
When the absorb cycle is finished/ended...

Float, in this stage the voltage is held above the resting fully charged voltage, but not enough to allow for significant charging, usually around 13.6-8 volts (12 volt) , 27.2- 27.6 for 24 volt systems. if a strong load is applied, say your running your table saw, the charge controller will allow enough of the energy coming from the array to maintain the set voltage point. So you will be in essence running off the solar array, not off the battery bank.

Equalizing would be done only on a fully charged battery, this is a controlled over charging of the battery, voltage is held a bit above the absorb level, around 30.6-32 volts in a 24 volt system. If one cell hasn't fully charged while the others have you will see an imbalance in the electrolyte (when measuring the Specific Gravity - SG), if this difference is more than .02-.025 you would want to do a corrective equalizing. The regular monthly maintenance equalizing will help keep the cells balance, but it's normally only done for a couple hours to keep the electrolyte from stratifying and being sure all the cells have fully charged. A corrective equalizing is done when you have a large imbalance, and you would measure the SG hourly until you have 2 hours with out an increase in SG, or the SG is brought to parity with the other cells.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Solar Expert Posts: 9,583 ✭✭✭✭✭
- submersible well pump
I use my tablet to stream movies, series, etc. from Netflix and Amazon.

Tablet - where does internet come from - don't forget to add the power of a sat RX or your cable modem, in 24 hours, it can be as much as 2 minutes of the microwave

Microwave.   Now we are talking inverter size.   Small 500w inverters are nice, low overhead self-consumption.  But getting to a 2KW inverter, idling along at 60 watts for couple lights and charger, you are burning a lot of power keeping the fuse warm !  May be a time to consider a dual inverter system, where you manually start the Big inverter for the Big jobs only.

Well Pump.   Do you know the motor size and power specs ?   Can you pump to above ground storage to reduce the times the big pump runs.

Most AC pumps will fall closely in line with this chart

Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Solar Expert Posts: 2,084 ✭✭✭✭✭
edited February 2017 #19
Staying with a 12 volt system will , if nothing else, limit your solar array size into your charge controller. At 24 volts you can double the wattage, at 48 volt you can quadruple the wattage. You will find yourself buying two or more charge controllers if you need the power that comes with a larger array to maintain that high amperage battery bank. There are 4 volt batteries available if you want to keep your battery count low.     http://www.trojanbattery.com/product/ind23-4v/

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

• Solar Expert Posts: 1,020 ✭✭✭✭
Must not forget, all those days of autonomy come at a price.  Will you have enough array to re-charge in 1-2 days?  Read up on deficit charging.  I think that striving for more days of autonomy and not using your generator will send you to the battery "bank" in a short time.  ie needing to replace an expensive battery investment sooner than necessary.  Generator fuel is cheaper than batteries.
• Registered Users Posts: 4,496 ✭✭✭✭✭
The pump is going to drive a lot of your choices. Is it 120v or 240v? Single phase? How much current to start?

The voltage matters for a couple of reasons relating to the basic P=V*I formula. For a given power, lower voltage means higher amperage. 2400 watts at 12v is 200a, at 48v it's 50a. To handle 200a, wiring etc has to be much bigger than 50a. Even if you wanted to spend the money on big cabling for high currents, inverter makers tend not to.

The two inverter system mentioned above may make sense. You already have a small 12v system for dc fridge. You could add a small PSW 12v inverter for small constant loads like lighting, and just use a big inverter for occasional large loads. I did this and it works well.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Registered Users Posts: 29 ✭✭
Ralph Day said:
Must not forget, all those days of autonomy come at a price.  Will you have enough array to re-charge in 1-2 days?  Read up on deficit charging.  I think that striving for more days of autonomy and not using your generator will send you to the battery "bank" in a short time.  ie needing to replace an expensive battery investment sooner than necessary.  Generator fuel is cheaper than batteries.
My perspective on all this is influenced by the behavior of the small solar-electric system I have now. Most of the time, I don't allow the SOC to go below 50%, and I have about 3 or 4 days of autonomy with this system. On a sunny day - even a short winter day - the batteries will charge to 100%. Granted, this system has only been up and running since last November, so I can't yet form any conclusion about whether the batteries will be healthy long-term. Note that most people would say my small system is over-paneled. It is, by most people's standards. I did this on purpose.

You ask, "Will you have enough array to re-charge in 1-2 days?" That's the thing: I am committed to having enough array to recharge in 1-2 days. I'm not going to cheap out on the array.
• Registered Users Posts: 29 ✭✭
Thanks to everyone for their contributions. They are very much appreciated.

I have one more question now; then I am going to hold off on future posts until I do some more research and studying so I can gain a better understanding of some of the info that has been provided in this thread.

The one question I have now relates to ventilation of the battery bank. As Photowhit said, I'll need to ventilate it. I do have one concern about the release of hydrogen from the battery bank.

I often operate a propane space heater and a kerosene convection heater In the basement area where the battery bank will be located. Of course, each of the two heaters has a open flame. Will it be safe to operate the two heaters near the battery bank, provided that I ventilate the battery bank properly?
• Registered Users Posts: 4,496 ✭✭✭✭✭
It would probably be a good idea to build a box to hold the batteries and ventilate the box to outside. The box will not only contain gasses, it also contains any electrolyte spray and prevents accidents like dropping a metal tool across battery posts.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter