Battery tools

What size battery do I need?

Work backwards: state the average load you want to carry and how many hours you want it to last, and the calculator returns the nominal capacity required once losses are added back in.

What size battery covers a 500 W load for 12 hours?

A 500 W average load for 12 hours needs 6.00 kWh delivered at the outlet. Because a battery cannot be fully discharged and the inverter loses energy converting DC to AC, the pack has to be larger: at 90% usable depth of discharge and 92% inverter efficiency you need about 7.25 kWh of nominal capacity. Sizing on the outlet figure alone would leave you roughly 20% short.

Your inputs

Average load to keep running

Hours you want to cover

Usable depth of discharge

Inverter efficiency

Reserve held back

Extra energy you want left in the pack when the target hours are up. Applied after the depth-of-discharge limit, not on top of it.

Scenario: hours of backup

7.2 kWh

of nominal battery capacity

6.00 kWh has to reach your appliances, so the pack must be larger than that

Calculated from your inputs

Load: 500 W
Target runtime: 12 h
Needed at outlet: 6.00 kWh
Nominal needed: 7.25 kWh

Method

0.5 kW × 12 h = 6 kWh needed at the outlet
6 kWh ÷ (0.9 usable depth of discharge × 0.92 inverter efficiency) = 7.246 kWh of nominal battery capacity

Assumptions

Usable depth of discharge 90% and inverter efficiency 92%.
No reserve held back.
The load is treated as constant across the whole period.
No solar or generator recharging during the outage.
Battery ageing and cold-weather capacity loss are not included.

What can change the result?

Whether your load average is realistic — most people overestimate it by summing nameplate ratings
How deeply the system actually allows discharge
Cold temperatures, which reduce usable capacity
Any recharging from solar during a daytime outage

Required nominal capacity by load and target hours

90% usable depth of discharge, 92% inverter efficiency, no recharging during the period.
Load profile	Average load	8 hours	12 hours	24 hours
Fridge, freezer and comms only	250 W	2.42 kWh	3.62 kWh	7.25 kWh
Essentials for a small home	500 W	4.83 kWh	7.25 kWh	14.5 kWh
Essentials for a typical family home	690 W	6.67 kWh	10.00 kWh	20.0 kWh
Essentials plus heating or cooling	1500 W	14.5 kWh	21.7 kWh	43.5 kWh

Getting the load figure right matters more than the maths

The arithmetic here is trivial; the input is not. Most oversizing comes from adding up nameplate ratings as if everything ran continuously. A refrigerator rated 150 W contributes about 50 W to a real average because the compressor cycles. Lights are on for part of the evening, not all day. The backup builder exists specifically to produce a defensible average rather than a worst-case sum.

Also decide honestly how long you need. Most grid outages are short; sizing for a rare multi-day event roughly triples the battery cost. Many households are better served by covering 8 to 12 hours well and accepting that a very long outage means running reduced loads.