# Sizing my battery bank

Options
Registered Users Posts: 19 ✭✭
I have a greenhouse with 10 dc fans and 4 panels.

The 4 285 watt 24 volt panels are wired in 2 series for 48 volts.

My fans total 8 amps. Correct my math if Im wrong. Im looking for 20 hrs worth of energy stored.
8 amps x 20 hrs = 160 ah x 2 (50% discharge) = 320 amp hours needed at 48 volts.

Is this a correctly sized battery bank?

Options
The 4 285 watt 24 volt panels are wired in 2 series for 48 volts.

My fans total 8 amps. Correct my math if Im wrong. Im looking for 20 hrs worth of energy stored.
8 amps x 20 hrs = 160 ah x 2 (50% discharge) = 320 amp hours needed at 48 volts.

Yes, basically correct if you want to run the fans 20 hours straight (no solar/generator/utility power). By the way, is this 10 hours per day with two days of back up power? Or do you run the fans 20 hours out of 24 hours per day? Do you run 9 months a year (no winter)? Or do you run during winter (low amounts of sun)? Where is the system installed (amount of sun by season)?

Charging, lets do some quick rule of thumb checks. First, you want to charge the battery bank at 5% to 13% rate of charge. If this is an "emergency power system", you might get away with a 5% rate of charge--However, if this is a daily use system, you probably want a larger array:
• 320 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,226 Watt array minimum
• 320 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,452 Watt array nominal
• 320 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 3,188 Watt array "cost effective" maximum
Next, sizing the array on the amount of sun you get... Guess 3 hours per day (early spring):
• 8 amps * 48 volts * 10 hours per day * 1/0.61 DC system eff * 1/3.0 hours per day = 2,098 Watt array minimum (10 hours per day operation)
• 8 amps * 48 volts * 20 hours per day * 1/0.61 DC system eff * 1/3.0 hours per day = 4,197 Watt array minimum (20 hours per day operation)
And then, when charging the battery bank--If the fans are operating during daylight hours, you need to account for the solar array providing both charging current and fan operational current... For 10% rate of charge:
• (320 AH * 0.10 rate of charge + 8 amps) * 59 volts charging * 1/0.77 panel+controller derating = 3,065 Watt array minimum (based on 10% rate of charge)
Whether you pick 5%, 10% or 13% rate of charge--Your choice... The deeper you discharge the battery bank (longer run times, bad weather, "winter sun", etc.)--You should be more towards 10% or higher rate of charge. If most of the time you run the fans primarily when the sun is up, not too many days of bad weather, non-winter operation, then you may be OK in the 5% to 10% rate of charge.

If you run the fans 10 hours per day--then you discharge the battery bank 25% per day--Very common usage for off grid. If you run the fans 20 Hours per day, that is 50% per day discharge--That can be quite difficult to recharge a lead acid battery bank back in one day... That can easily be 10+ hours per day of charging, and, you may only get 6 hours per day of sun during "non-summer" operation. If you operated winter/early spring 20 hours per day and want to run fans during cloudy weather--Then you might need to do a 2x larger AH battery bank (or choose another battery chemistry/other way of running ventilation/cut back on cloudy days, etc.).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset