Set up info

notes

First I would like to thank all for info on this site and the help you have all given me. After months of monitoring our loads I have come up with 2382 wh per day. This is a weekend cabin 2-7 days off grid 6 miles off asphalt and any power, in northern Michigan. Would like your input on equipment before I talk the next step in purchasing. Major surges are shallow well pump, freezer and occasional use of m/w. I am sizing battery bank for 2 days and using 5kw split phase generator for charging and generator support. Solar panels in near future.
Battery bank @ 24 volts 490 ah. @ 48 volts 245ah. Panels wattage estimate at 2000 -2440 watts. Inverter choices, outback radian GS4048, magnum MSH4024RE, schneider XW 4024 or 4548. I would need a transformer for generator using magnum. Any comments appreciated good or bad.

BB.

Re: Set up info

notes wrote: »

First I would like to thank all for info on this site and the help you have all given me. After months of monitoring our loads I have come up with 2382 wh per day. This is a weekend cabin 2-7 days off grid 6 miles off asphalt and any power, in northern Michigan. Would like your input on equipment before I talk the next step in purchasing. Major surges are shallow well pump, freezer and occasional use of m/w. I am sizing battery bank for 2 days and using 5kw split phase generator for charging and generator support. Solar panels in near future.
Battery bank @ 24 volts 490 ah. @ 48 volts 245ah. Panels wattage estimate at 2000 -2440 watts. Inverter choices, outback radian GS4048, magnum MSH4024RE, schneider XW 4024 or 4548. I would need a transformer for generator using magnum. Any comments appreciated good or bad.

Lots of good people here volunteering their time and experiences.

Assuming your needs are correct (2,382 WH per day sounds a bit small for all of your loads--But it is certainly doable), I would look a little bit at the capabilities and "balance" of your system.

A 490 AH @ 24 volt battery bank would really only "reliably" drive a ~ 2,450 Watt inverter (~100 AH @ 48 volts per 1,000 Watt of inverter or 200 AH @ 24v or 400 AH @ 12v). A 4-4.5 kWatt inverter is pretty large for a flooded cell battery bank (if other type like AGM or LiFePO4 or etc.--They can supply a lot more surge current--If you loads are generally light, but you need occasional large surges support).

Start with a full time rule of thumb system design. You could cut the battery bank AH by 1/2 (25% discharge per day vs 50% discharge per day) for a seasonal/weekend cabin:

• 2,382 WH per day load * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 467 AH @ 24 volt battery bank

Then to charge such a bank using 5% to 13% rate of charge (use your 490 AH @ 24 volt bank numbers):

• 490 AH * 29 volts charging * 1/0.77 panel+controller losses * 0.05 rate of charge = 922 Watt array minimum
• 490 AH * 29 volts charging * 1/0.77 panel+controller losses * 0.10 rate of charge = 1,845 Watt array nominal
• 490 AH * 29 volts charging * 1/0.77 panel+controller losses * 0.13 rate of charge = 2,399 Watt array "cost effective maximum"

And based on how much sun and seasonal usage (where will cabin be? no shading?) -- Lets use 4 hours of sun as a good ~9 months a year generator "Break Even" sizing:

• 2,382 WH per day * 1/0.52 typical end to end system eff * 1/4 hours of sun per day = 1,097 Watt solar array (4 hour per day sun)

And charging the battery bank with an AC genset--Worst case assumptions (poor power factor, 80% of genset rated output)

• 490 AH * 29 volts charging * 0.10 rate of charge * 1/0.80 charger eff * 1/0.67 charger PF * 1/0.80 genset derate = 3,314 Genset VA rating
• 490 AH * 29 volts charging * 0.20 rate of charge * 1/0.80 charger eff * 1/0.67 charger PF * 1/0.80 genset derate = 6,628 Genset VA rating

Or based on a typical 5kW genset:

• 5,000 VA * 1/29 volts charging * 0.80 charge eff * 0.67 PF * 0.80 Genset derate = ~74 amp @ 24 volt battery charger

If your charger is more efficient (typically an Inverter+Charger), you could drive as much as:

• 5,000 VA * 1/29 volts charging * 0.90 charge eff * 0.9 PF * 0.80 Genset derate = ~112 amp @ 24 volt battery charger

So... Looking at your numbers--Other than using a 4kWatt AC inverter, your system seems to be designed towards the rules of thumb for a full off grid 9+ month per year home based on your estimated 2,382 WH per day loads.

Looking at the capabilities of a 490 AH battery bank:

• 490 AH * 24 volts * 0.85 * C/20 hour rate = 500 Watt average load (5 hours per night for 2 night, 50% max discharge)
• 490 AH * 24 volts * 0.85 * C/8 hour rate = 1,250 Watt average maximum continuous load (over ~4 hours)
• 490 AH * 24 volts * 0.85 * C/5 hour rate = 1,999 Watt maximum continuous load (over ~few hours)
• 490 AH * 24 volts * 0.85 * C/2.5 hour rate = 3,994 Watt maximum surge load (seconds to a few minutes)

Most "good quality" AC inverters will supply approximately 2x rated load--Or in this case, a ~2,000 Watt inverter will supply ~4,000 watts for seconds to a few minutes.

So--Is this all what you expected (other than the 4-4.5 kWatt AC inverter)?

Over-sizing an AC inverter generally has higher standby and operating power (just using power to "turn on" or tare losses).

Generally, there is not usually a good reason to 2x over size an AC inverter unless you have a lot of surge current/short term-high current loads. And flooded cell lead acid batteries are not great at really high surge currents (C/2.5 or so)... AGM and Lithium can supply C*1 or even C*3 or C*4 surge currents. However, for AGM, these high currents are usually seen on UPS type systems that draw the batteries dead in 15-30 minutes or so--And battery life after a few of these cycles is usually not great.

LiFePO4 are better for high current (charging and discharging) needs--But may be a bit cutting edge for your needs. Although, there are starting to be some very good arguments to try them (should have longer life and lower life cycle costs, which reduces the initial higher purchase costs).

-Bill

Cariboocoot

Re: Set up info

notes wrote: »

First I would like to thank all for info on this site and the help you have all given me. After months of monitoring our loads I have come up with 2382 wh per day. This is a weekend cabin 2-7 days off grid 6 miles off asphalt and any power, in northern Michigan. Would like your input on equipment before I talk the next step in purchasing. Major surges are shallow well pump, freezer and occasional use of m/w. I am sizing battery bank for 2 days and using 5kw split phase generator for charging and generator support. Solar panels in near future.
Battery bank @ 24 volts 490 ah. @ 48 volts 245ah. Panels wattage estimate at 2000 -2440 watts. Inverter choices, outback radian GS4048, magnum MSH4024RE, schneider XW 4024 or 4548. I would need a transformer for generator using magnum. Any comments appreciated good or bad.

Which salesman have you been listening to?

Read my signature line. That tiny system supports about the same Watt hour need as you have, without any fancy gen support or 240 VAC split-phase inverters. My battery bank is half the size as is the array (which was expanded this year; it was 700 Watts before).

Seriously. 2400 Watt hours per day is 100 Amp hours on 24 Volts. At 25% DOD you'd do that on around 400 Amp hours, especially if at least some of the load can be handled 'directly' by panels. 1250 Watts of array would provide the necessary 40 Amps peak charge current.

You wouldn't really need a transformer for the gen either, as the Watt demand for charging is less than 50% of its maximum. Switch the loads on to one side and the charger on to the other if you are worried about balance. Or get a smaller gen since you won't really need 5kW.

Do you really need a 4kW inverter? I doubt it. My 3.5 is rarely loaded to 1/3 its capacity. The biggest thing on it is the 1 HP septic pump. Second would be the microwave which is used infrequently and only when the power is there. I can Bulk charge in the morning for an hour or two and then use the power tools all day, including the 1200 Watt air compressor.

I don't ordinarily say this, but I think you're planning on buying too large a capacity system.

notes

Re: Set up info

Thanks Bill and Cariboocoot. No salesman, all on myself. I think I over figured. More IS NOT better. I will look at smaller inverter more in line with my usage and less battery. Again thank you for all your input and advise.

vtmaps

Re: Set up info

Cariboocoot wrote: »

Read my signature line. That tiny system supports about the same Watt hour need as you have, without any fancy gen support or 240 VAC split-phase inverters.
<snip>
I don't ordinarily say this, but I think you're planning on buying too large a capacity system.

I'm not so sure about that... It all depends on lifestyle. How much do you want to manage your energy use?

Its breakfast time. Batteries are at their low point of the day. Start the microwave. Freezer cycles on. Water pump cycles on. Lights (and everything else) go out.

Cariboocoot (and I) manage our power use carefully... his 1 hp septic pump doesn't run on automatic... he turns it on manually when he has enough power.

I too have a small battery bank and manage my loads... I do laundry or vacuum only while the sun shines or the generator hums. No high current draws from RV style 12 volt water pump and DC fridge and freezer. No microwave.

--vtMaps

notes

Re: Set up info

Thanks all, the more I read the more confused I get. Bill, are you saying a 24 volt system needs a 200ah battery bank per 1000 watt inverter? How critical is that? If I read through most of everyone's sig, all are under sized. I am not disagreeing, just trying to understand. At a maximum I guess I should be looking at a 2500-2000 watt inverter and cut the 490ah bank in half then, provided my loads our correct?

BB.

Re: Set up info

More or less--For a reliable system (works over time and various state of charge conditions), the maximum sustained discharge from a Flooded Cell battery bank is around C/5 (Battery AH capacity we use around here is the 20 Hour discharge rate--I.e., the current needed to discharge a battery to 0% in 20 hours).

So, the maximum sustained current would be

• 490 AH * 24 volt battery bank * 0.85 AC inverter eff * 1/5 discharge rate = 1,999 Watt AC load (inverter rating)

And the maximum surge current would be based on a C/2.5 discharge rate (in theory, discharge the battery dead in 2.5 hours):

• 490 AH * 24 volt battery bank * 0.85 AC inverter eff * 1/2.5 discharge rate = 3,998 Watt AC load (inverter surge rating)

Other battery types (AGM, Lithium, etc.) support higher rates of discharge--So, in theory, you could run a larger AC inverter.

However--The AH capacity is still the same--So, if you have other lower power, long term loads, you will still drain a 490 AH @ 48 volts the same whether the "cheaper" Flooded Cell Lead Acid or the more expensive AGM/Lithium types.

These numbers are very rough... Hot batteries can output more energy/surge current. Cold batteries less. Fully charged batteries can output more surge current than those at 50% or less state of charge. Older sulfated batteries have less exposed active plate material and lower specific gravity (sulphate permanently bonded to lead as hard black sulfate crystals).

This numbers are conservative/rough guidelines to do an initial system design and end up with a "balanced" system (and hopefully optimum use of money).

Can you run 4kWatts out of new fully charged lead acid battery--Probably yes. Could you pull 4kWatt from a 6 year old battery flooded cell bank at 50% state of charge on a cloudy day--Perhaps not.

For Example: If you have needs for a 4kWatt AC inverter and want to try a 490 AH @ 24 volt battery bank--It probably will work pretty well. Most people will find it difficult have a load accurately pull "exactly" 8 kWatt surge on a 24 volt battery bank:

• 8,000 Watt * 1/0.85 inverter eff * 1/21 volts minimum inverter voltage = 448 Amps of surge current

That is a lot o current and a lot of voltage drop on any reasonable sized Inverter/Battery Bank wiring.

-Bill

notes

Re: Set up info

Thanks for the explanation Bill. Regarding your rv pump. Do you use a storage tank and use the pump for pressure? Will it lift from a 10' shallow well. My biggest surge is my pump. I did change out motor with a 3wire capacitor start motor witch seemed to drop my start amps quite a bit.

Cariboocoot

Re: Set up info

Two points: anyone expecting to run an 8 kW load from a 24 Volt system had better be prepared to be disappointed, and anyone running their 24 Volt system down to 21 Volts better have half ownership in a battery company.

BB.

Re: Set up info

That does 21 volts does include 1-2 volts of voltage drop in the wiring/breaker/fuse/switch too.

-Bill