How big should my solar battery be?

Updated October 2026

A battery is sized in kilowatt-hours (kWh) of energy, not in kilowatts. The answer depends on what you want it to power and for how long, and on the battery type, because some batteries can only be used down to half full. The batteries page compares the types side by side.

Step 1: choose your backup goal

• Hybrid home with outages: cover the essentials, for example 40 percent of daily use for 1 day.
• Off-grid: cover all daily use, 100 percent, for 1 to 3 days without good sun. This guide uses 2 days where noted.

Step 2: calculate useful energy

Useful kWh = daily use × backup share × days of backup ÷ 0.9. The 0.9 covers losses in the inverter and battery. Example with a daily use of 29.6 kWh (900 kWh a month): 29.6 × 0.4 × 1 ÷ 0.9 = 13.2 kWh.

Step 3: allow for depth of discharge

Nominal size = useful kWh ÷ usable depth of discharge. Lead-acid batteries last much longer if you use only about half their rating, while lithium iron phosphate can be used much deeper.

So the same 13.2 kWh need means about 14.7 kWh of lithium iron phosphate or about 26.4 kWh of lead-acid. For a fully off-grid home using 2 days of backup, the useful need is 29.6 × 2 ÷ 0.9 = 65.8 kWh, so plan on roughly 73 kWh of LiFePO4.

Check these too

• Power (kW): the battery and inverter must supply your peak load, including the start-up surge of pumps, fridges and air conditioners.
• Voltage: 12, 24 or 48 V. Higher voltage means lower current and thinner cables. Match the battery voltage to the inverter.
• Cycles and cost: lithium costs more up front but gives several times more cycles, so compare cost per cycle.

Next step: compare battery types and size the whole system.