Battery Bank Sizing

Work backwards from the load, for the worst month.

The method

Solar sizing runs backwards from the load, and this calculator is that method with the arithmetic done:

• Battery watt-hours = daily load × days of autonomy ÷ depth of discharge. The tank is bigger than the part of it you may use: about 80% of a LiFePO4 bank is spendable, about 50% of lead-acid.
• Array watts = daily load ÷ worst-month sun hours ÷ derating. The array's job is to replace a day's spending during the stingiest month's sun. Derating (0.7 to 0.8) is the honest discount for angle, heat, dust, wiring, and controller losses.
• Controller amps = array watts ÷ system voltage × 1.25. Round up to the next size the market sells; a size up again is cheap headroom for panels you add later.

Size for your worst month and summer takes care of itself. An array sized on the yearly average hides December inside June and leaves the battery drained for weeks at exactly the wrong time of year.

Getting honest inputs

• Daily load: watts × hours for every device, measured where possible. Duty cycle counts: a 60 W pump that runs 90 minutes is a 90 Wh load. Inverter standby counts too, all 24 hours of it.
• Days of autonomy: two to three is the usual farm answer for how many sunless days storage alone must cover.
• Worst-month sun hours: a December number, not a June one; two is a fair mid-latitude default.

The full reasoning, and a worked stock-tank example, lives on the site's Sizing a System page; battery chemistry and its manners on Batteries and Storage.

Free under CC BY 4.0. Cite as "Open Agriculture Technology Battery Bank Sizing Calculator (openagriculturetechnology.com)".