1.0 kWexample average essential load
8.7 kWhusable battery after inverter loss
10.7 kWhnominal with DoD and reserve
Battery size by average essential load
| Average essential load | 8-hour delivered energy | LiFePO4 nominal | Lead-acid nominal |
|---|---|---|---|
| 0.5 kW | 4.0 kWh | 5.4 kWh | 9.7 kWh |
| 1.0 kW | 8.0 kWh | 10.7 kWh | 19.3 kWh |
| 1.5 kW | 12.0 kWh | 16.1 kWh | 29.0 kWh |
| 2.0 kW | 16.0 kWh | 21.5 kWh | 38.6 kWh |
The table matches the calculator defaults: 92% inverter efficiency, selected depth of discharge, and 10% reserve. Temperature, aging, surge, and code constraints remain outside this energy-only estimate.
Formula and worked calculation
nominal battery kWh = (load kW x hours) / inverter efficiency / depth of discharge / (1 - reserve)
For a 1.0 kW average essential load: 1.0 x 8 = 8.0 kWh delivered. Then 8.0 / 0.92 / 0.90 / 0.90 = 10.73 kWh nominal. A practical product selection may round above that number.
Build the load from appliances, not the main breaker
- Refrigerator: use measured average energy plus compressor starting surge.
- Lights, router, electronics, and fans: add watts for the hours each item will run.
- Well pump, sump pump, air conditioner, heat pump, or electric heating: check inverter surge and continuous power separately from battery kWh.
- Do not assume a whole home consumes its utility-service rating continuously.
Country and regional parameters to check
- United States: verify transfer equipment, critical-load panel design, local code, and whether the battery can operate during grid outage.
- Australia: verify inverter approval, backup-circuit limits, network requirements, and heat derating.
- United Kingdom: verify EPS/backup output, installer design, winter load, and equipment certification.
Error boundary: energy capacity alone does not prove that a battery can start a pump, compressor, or motor. Confirm inverter continuous kW, surge kW, transfer behavior, battery current limits, temperature range, and code requirements.
Run the battery estimate, then check the solar side
The prefilled calculator represents 8 kWh of essential outage energy for one planning period. Its result tools provide a reproducible link and Solar Project Summary.
Related planning paths
Worked outage scenarioReview overnight home backup
Off-grid scenarioReview an 8 kWh/day cabin
System comparisonCompare solar system sizes
Last reviewed July 18, 2026. Planning estimate only; no equipment compatibility or installation result is promised.