Home Battery Runtime Calculator
Calculate approximate backup runtime from battery capacity, usable depth, inverter efficiency and average load.
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How to choose the inputs
Enter the capacity basis first. If capacity is nominal, use the fraction available for this outage. If it is already fully usable energy, use 100% unless you deliberately retain an additional reserve. Count average simultaneous loads rather than adding every appliance nameplate.
Formula and measurement boundary
Three worked scenarios
These are fixed hypothetical examples, not live tariffs or verified product data. They use EUR and kilometres where applicable and do not change with the form settings. Only the table input changes.
Unchanged inputs
- Battery capacity: 10 kWh
- Usable capacity: 90 %
- Inverter efficiency: 90 %
| Average load | Estimated result |
|---|---|
| 300 W | 27 h 0 min |
| 600 W | 13 h 30 min |
| 1,200 W | 6 h 45 min |
These inputs provide 8.1 kWh to the loads. Doubling the average load halves runtime. A refrigerator cycling on and off should be represented by its measured average, not continuous compressor power.
What this result does not establish
- Energy capacity does not prove the inverter can start or power the equipment.
- Idle consumption, aging, temperature and power-dependent efficiency are not modelled separately.
- Grid-connected storage needs supported backup hardware to supply loads during an outage.
Build a critical-load plan before estimating whole-house runtime
The following are additional hypothetical decision examples. Separately discussed fees, reserve allowances or running costs are not automatically added to the form result.
A hypothetical 10 kWh battery with 80% available to use and 90% conversion efficiency supplies 7.2 kWh to loads. At a steady 300 W this lasts 24 hours; at 1.5 kW it lasts 4.8 hours. These loads describe two different backup plans. Removing electric space heating from the backed-up circuit can matter more than buying a slightly larger battery.
Separate appliances that must stay on from tasks that can wait. Measure a refrigerator over a full operating cycle rather than treating its compressor rating as a continuous load. Include the inverter’s own demand if it is not already included in your measured load. For example, a 25 W idle draw consumes 0.6 kWh over 24 hours, a meaningful part of the low-load scenario.
Energy capacity does not establish that an appliance can start or that the house can operate during an outage. Verify continuous output, motor surge, supported circuits and transfer/islanding equipment with the installer. A grid-connected solar array alone is not a backup system. Run the calculator again for night-time operation, when solar replenishment is zero.
Sources and their scope
- EIA: measuring electricity — Power and energy units only; not equipment performance.
These reference pages were accessed on 21 September 2026. The equations and examples are the site’s own model; these sources do not certify its defaults or results.
Frequently asked questions
Can I use this to size an inverter?
No. Check continuous and start-up power separately against the equipment documentation.
Should I apply usable capacity twice?
No. Use the usable-share input only for a reduction not already included in the entered capacity.
Read the model and verification limits