inverter size vs battery

For a 210 AH @ 24 volt battery... Use C/5 as the maximum inverter and C/10 as the "comfortable" inverter (for flooded cell lead acid battery capabilities):

• 210 AH * 24 volts * 0.85 AC inverter eff * 1/5 hour discharge = 856 Watt inverter max suggested
• 210 AH * 24 volts * 0.85 AC inverter eff * 1/10 hour discharge = 428 Watt inverter "nice" size

The other rule of thumb, for a 24 volt battery bank, ~500 Watt inverter (and suggested maximum array wattage) per 100 AH @ 24 volts:

• 210 AH battery bank * 500 Watts * 1/100 AH = 1,050 Watts

AGM, GEL (and Li Ion) can surge more current and support a larger AC inverter... But you sort of want to avoid taking a battery bank "dead" in ~5 hours (or less), or to 50% state of charge in 2.5 hours or less for a home/cabin.

When you compare to, for example, 5 hours at night for 2 nights, with 50% maximum discharge, the average load would be:

• 210 AH * 24 volts * 0.85 AC inverter eff * 1/2 days * 0.50 max discharge = 1,071 WH per day average battery load
• 1,071 WH average battery load / 5 hours per day (evening) usage = 214 Watt average loads for 120 VAC

And why a 300 Watt inverter with 600 Watt surge is not a bad fit for a small system.

Of course, these are really rough rules of thumbs (C/5 and C/10 discharge rates)--But do get you a pretty reliable system.

-Bill

Around 1,200 to 1,800 Watts is about the maximum I would suggest for a 12 volt battery bank:

• 1,800 Watts * 1/0.85 AC inverter eff * 1/10.5 volts battery cutoff = 202 Amps
• 202 Amps * 1.25 NEC derating (to lessen the chance of blowing fuses/breakers) = ~252.5 Amp rated branch wiring+breaker/fuse

And if you look at voltage drop, trying to keep below ~0.5 volts (12 volt bus). Using the NEC cable and ~250 Amps would be a 4/0 cable.
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm
Using a voltage drop calculator, the one way run for 202 amps @ 0.5 volt drop @ 4/0 cable would be:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.1608&voltage=12&phase=dc&noofconductor=1&distance=25&distanceunit=feet&amperes=202&x=0&y=0

In this example, you could go out to ~25 feet (0.5 volt drop)--Although, you probably would generally be much shorter cables...

Same thing at 24 volts:

• 1,800 Watts * 1/0.85 AC inverter eff * 1/21.0 volts battery cutoff = 101 Amps
• 101 Amps * 1.25 NEC derating (to lessen the chance of blowing fuses/breakers) = 126 amp minimum rated wiring+breaker/fuse

That would be ~2 AWG cable... And the voltage drop at maximum one way run @ 1.0 volt drop is ~30 feet (again, keep inverter wiring short).

If you go with a 420 AH @ 24 volt battery bank (~2,200 Watt inverter suggested maximum), the array size based on battery usage would be:

• 420 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 791 Watt array minimum
• 420 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,582 Watt array nominal (full time off grid min)
• 420 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,056 Watt array "typical" cost effective maximum

Your 2,000 Watt array looks good with this bank. A 12 volt bank would be 840 AH @ 12 volts... That is about the largest AH battery bank I would suggest (heavy wiring, need >80 Amp charge controller for 10% rate of charge, etc.).

So, yes, a 24 volt battery bank looks better--And unless you really need 12 volts (HAM radio, other "12 volt devices")...

-Bill