2100 ah bank not enough solar array

Re: 2100 ah bank not enough solar array

Everything vtMaps says...

Be very careful here--You are speaking of two systems--And confusing me between the two systems. And not using paragraphs to separate thoughts/systems.

I will "fix" your paragraphs to make it easier to read/understand.

And a 4 kW inverter on 420 watt array--That is an "unbalanced" system. Your large inverter is drawing around 20-60 watts just to "turn on" with no other AC loads. Say 25 watts of "tare" losses:

24 watts * 24 hours always on inverter * 1/0.52 end to end losses * 1/4 hours of sun = 277 watt array minimum just to run inverter 24 hours per day

And there is the 5% to 13% rate of charge for a 435 AH @ 24 volt battery bank:

• 435 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 819 Watt array minimum
• 435 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,638 watt array nominal
• 435 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,130 watt array "cost effective maximum"

Plus, battery banks have maximum "practical" capabilities... Another rule of thumb would be a maximum inverter sizing of ~0.5 kW per 100 AH @ 24 volt battery capacity... Or:

• 435 AH * 0.5 kW * 1/100 AH (at 24 volt) = 2.175 kW = 2,175 Watt practical (reliable) maximum AC inverter output for this battery bank

With solar power--The old real-estate adage of Location, Location, Location applies. Guessing your location is somewhere around Concord New Hampshire, using PV Watts for a fixed array tilted (43 degrees from horizontal) to latitude:

Month    Solar Radiation (kWh/m 2/day)
1      3.80     
2      4.56     
3      4.89     
4      5.19     
5      5.40     
6      5.42     
7      5.59     
8      5.47     
9      4.93     
10      4.01     
11      3.13     
12      2.87     
Year      4.61

Tossing the bottom 3 months as (you will have to use the generator)--Then the typical amount of "average sun" (+/- ~10% over 20 year average) would give you a "break even" solar/generator use month of October at 4.01 hours.

A 420 watt array (ignoring significant inverter tare losses) would be around:

• 420 Watt * 0.52 end to end system losses * 4.01 hours of sun = 876 Watt*Hours = 0.9 kWH per day from solar in average October

I can believe you are using a lot of generator run-time... And a lot of fuel. There is a whole set of questions/calculations on generator sizing to loads/battery bank too... Just like driving a vehicle and figuring out your best fuel economy, there is something similar to balance your electrical loads to generator sizing and operating algorithm.

More or less, a standard genset gets best fuel economy if you run it at 50% or more of rated load, and gets very poor fuel economy at 25% or less load (i.e., use 2x or more fuel than you really need to). Sometimes people end up with two generators--A large one to "bulk the battery bank" and run shop tools... And a second, smaller genset to finish up charging and run a few loads (like a Honda 1.6-3 kWatt or similar Yamaha inverter-genset).

Lastly, which should be first--You really need to measure and log temperature corrected specific gravity for each cell in your battery banks. And make sure you are operating it in a way that will ensure long life for the battery bank. Usually something like recharging >90% State of charge a couple times a week, and avoid discharging below 50%, and don't let unused bank set below 75% SOC. Check water usage (watering every month maybe over charging; adding water every 6 months or less, probably undercharging, etc.).

Lots of reading:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

Anyway, at this point, more questions than answers right now.

-Bill