How this calculator works
Estimate backup-battery runtime from usable capacity, reserve setting, inverter efficiency, and average electrical load.
Usable capacity is smaller than nameplate capacity
A reserve setting, battery-management limits, inverter losses, temperature, and battery age can reduce the energy available to loads. The calculator applies reserve and inverter-efficiency inputs before estimating runtime.
Average load matters more than a single peak
Runtime is based on average watts over the backup period. A refrigerator, pump, or HVAC system may cycle and also have a higher starting surge. Verify that the battery and inverter can supply both continuous and surge power.
- Build a critical-load list.
- Measure or estimate duty cycles.
- Keep medical and life-safety planning separate from a casual calculator estimate.
Solar recharging changes the scenario
This calculator assumes no charging during the modeled runtime. Daytime solar production, generator charging, grid availability, and load shedding can extend the operating period.
Frequently asked questions
Why is calculated runtime shorter than capacity divided by load?
Reserve settings and conversion losses reduce usable AC energy, and real batteries may also be affected by temperature and age.
Does this check surge power?
No. Confirm continuous and surge ratings separately with the equipment documentation.
Can I use this for medical backup planning?
Do not rely on a general calculator alone. Follow equipment requirements and use a qualified backup-power plan with appropriate redundancy.
Use equipment documentation, your utility rate schedule, and measured data when they are available.