Battery Backup Calculator

Find how long your inverter batteries will last, or how many batteries you need for the hours of backup you want.

How to calculate battery backup time

Battery energy in watt-hours = volts × amp-hours × number of batteries.
Usable energy = battery energy × depth of discharge × inverter efficiency (about 90%).
Backup time in hours = usable energy ÷ load in watts.

Two 12 V 200 Ah tubular batteries store 2 × 12 × 200 = 4,800 Wh. Used to 50% through a 90% efficient inverter, that is 2,160 Wh, so a 300 W load runs for about 7.2 hours. The same capacity in lithium, used to 80%, gives about 11.5 hours.

How to calculate the number of batteries

Work out the energy your appliances use during the backup hours.
Divide by the depth of discharge and the inverter efficiency to get the energy the bank must store.
Divide by the system voltage to get the amp-hours needed.
Put enough batteries in series to reach the system voltage (two 12 V batteries for 24 V, four for 48 V), then add parallel strings until you have enough amp-hours.

Battery types

Tubular flooded batteries are the most common in Nigeria. They are robust and cheaper, but need distilled water topping and should only be discharged to about 50%. AGM and gel batteries are sealed and need no maintenance but have a shorter life in hot rooms. Lithium iron phosphate (LiFePO4) batteries cost more but give much more usable energy, recharge faster on solar and last two to three times as long.

Make your batteries last longer

Recharge fully as soon as possible after a long outage; lead-acid batteries left half empty lose capacity.
Keep the batteries in a cool, ventilated place, away from direct sun.
Check the water level of tubular batteries every month and top up with distilled water only.
Set the inverter's charging current and low-voltage cut-off to suit the battery type.

Frequently Asked Questions

Need the inverter too? Use the Inverter Size Calculator, or size panels, batteries and inverter together with the Solar System Calculator.