Battery sizing runs in two steps: work out the usable energy you actually need to deliver, then gross it up to the rated (nameplate) capacity, because you never draw a battery flat and the inverter loses some energy as heat. The rated figure is always bigger than the energy your appliances see.
Step 1: the usable energy
Multiply the load you want to run by the hours you want it to last: usable kWh = load kW × backup hours. A 500 W (0.5 kW) load for 8 hours is 4 kWh. Be honest about the load — add up only what will really run during an outage (lights, fridge, wifi, a few sockets), not the whole house.
Step 2: gross up to rated capacity
Divide the usable energy by the depth of discharge (how much of the battery you use) and the inverter efficiency. At 90% depth of discharge and 92% inverter efficiency: 4 ÷ 0.9 ÷ 0.92 ≈ 4.8 kWh of rated battery. Lower depth of discharge or a less efficient inverter means a bigger bank.
| Load | Backup | Usable kWh | Rated kWh* |
|---|---|---|---|
| 300 W (essentials) | 8 h | 2.4 | ~2.9 |
| 500 W | 8 h | 4.0 | ~4.8 |
| 1 kW | 5 h | 5.0 | ~6.0 |
*At 90% depth of discharge and 92% inverter efficiency.
Battery Bank Sizing
Enter the load, target backup hours and the size of one battery — it grosses up for losses and returns the total kWh and the number of batteries.
How many batteries?
Divide the rated capacity by the size of one module and round up. A 4.8 kWh requirement takes one 5 kWh battery; 6 kWh takes two. Round up to a whole battery — there is no half-battery to buy.
Add a real safety margin
These figures are the minimum for a best-case, steady load. Batteries fade with age and cold, and startup surges and forgotten loads add up. Round up to the next battery and keep some headroom rather than sizing to the exact number.
Don’t forget the recharge
A bank is only useful if it refills between outages. Make sure you have enough solar or grid charging to top it back up in time for the next one — a big battery with too little charging capacity spends most of its life half empty.