4080Watts of Solar, what's a good battery bank size

Rngr275 · April 2021

Going through a bunch of stuff Batteries and generator. I foresee batteries in my future. Current bank was 16 Trojan L16 RE, 370Ah. 8 in series (48V) and the other 8 in parallel (740Ah). Only ~50 amps available from the GenSet! Solar World 255W panels purchased in 2012. Trying to size the new back and looking for Maufaturer recommendations.
Thanks,
McD
RLTW

stillchillin · April 2021

Hi Ranger, I have a similar sized array connected to a 1350AH bank and am going on 11 years with the original bank. I know i'm a little under paneled and charge at about 10% but this has worked for me. I cant do the math without looking up some formulas like some of the really sharp folks here and I'm sure they will tell you exactly what you need, saw your shout out for info about your battery/ generator problems but was to busy to chime in. I too have had a long dark winter with lots of generator run time compared to years past but made it through just fine.
I am on my second Generac generator, the first was a 8KW when that needed a new armature in December and the part was $500 and 8 weeks out I upgraded to a 11KW unit, a much nicer unit as it has a 2 cylinder motor, easier starting when sub zero and smoother also quieter. I think I may have contributed to the failure by having the cooldown cycle to short, it still lasted 8 years at about 300 hrs per year.
oh by the way I'm in Vermont about a 1/2 hour from Canada. I would recommend the 11 KW generator to anyone with a similar size battery bank. Just my 2 cents hope this helps. Good luck

BB. · April 2021

The nominal range of charging is 5% (can work for weekend/sunny weather system...). 10%-13% good for full time off grid system (many/most? deep cycle lead acid battery vendors recommend 10% rate of charge... Can go as high as 20% if needed. 13%+ rate of charge, really should have a remote battery temperature sensor on the bank--Higher rates of charge can heat battery banks and shorten their life a bit). Note that we use 10% of the battery's 20 Hour rating (current capacity for discharge over 20 hours from full to dead). Some vendors (such as Rolls?) may use, for example 6 Hour discharge capacity (and different % rates of charge numbers).

Use ~77% derating for panels (Vmp falls as panels get hot in full sun, and 5% controller losses).

I use the "same math" for PWM and MPPT type charge controllers... Close enough for solar (assuming PWM controller has properly matched Vmp-array and Vbatt voltages).

For example, the math looks like:

740 AH * 59.0 volts charging * 1/0.77 panel+controller derarings * 0.05 rate of charge = 2,835 Watt array minimum
740 AH * 59.0 volts charging * 1/0.77 panel+controller derarings * 0.10 rate of charge = 5,670 Watt array nominal
740 AH * 59.0 volts charging * 1/0.77 panel+controller derarings * 0.13 rate of charge = 7,371 Watt array "typical" cost effective max
740 AH * 59.0 volts charging * 1/0.77 panel+controller derarings * 0.20 rate of charge = 11,340 Watt array "over-sized" array

Other issues--Do not forget these--You want to "meet" all the requirements. Battery bank big enough for daily loads, can recharge quickly, and supply high enough load and surge current. And also the trade off between hours of sun/array size vs genset+fuel usage...

Suggest battery bank is 2 days storage and 50% max planned discharge (4x daily energy seems to be optimum). 3 days x 0.50 max discharge works (6x daily energy usage).

And size the array based on daily loads and hours of sun per day (by season)...

-Bill

Rngr275 · April 2021

Thank both of you!

4080Watts of Solar, what's a good battery bank size

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