EngineerHub.orgWhat this calculator does
This single tool covers the questions that are often split across separate battery calculators: required stored energy (kWh), expected runtime, continuous and surge inverter power (kW), essential-load versus whole-home backup, and whether a candidate battery/inverter combination passes the screening checks.
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Simplified mode estimates average backup load from monthly kWh, then applies scope and household-peak multipliers. That method is less reliable than an appliance list and is reported as a range.
Energy capacity and inverter power are separate limits
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Sizing answers “will it work?”; economics answers “is it worth it?”
Backup sizing and financial sizing are different problems. A battery large enough for a 12-hour outage may be larger—or smaller—than the capacity that is economically useful for shifting solar energy or time-of-use tariffs. After checking kWh, kW and runtime here, use the Home Battery Payback & Solar Self-Consumption Calculator to estimate annual savings and the marginal value of adding more capacity.
Background
Optional reading — open any section below.
Battery energy is measured in kilowatt-hours (kWh) and determines runtime. Inverter power is measured in kilowatts (kW) and determines how many appliances can run at once. A system can have enough stored energy but still fail to start a pump if its inverter surge rating is too small.
For example, a 10 kWh battery feeding an average 1 kW load has roughly ten hours of ideal energy before depth-of-discharge and conversion losses are considered. A 3 kW inverter, however, cannot continuously supply a 5 kW load even if the battery is full.
Monthly consumption shows average energy use but does not reveal which appliances operate during an outage or the highest simultaneous load. Two homes can both use 900 kWh per month while one has steady small loads and the other has pumps, air-conditioning or electric heating.
The simplified mode therefore applies broad household-profile multipliers and reports a range. Use appliance-list mode before purchasing equipment whenever nameplate data are available.
If the detailed load list requires 6.0 kWh and a 15% reserve is added, the load-side requirement becomes 6.9 kWh. With 90% allowable depth of discharge and 92% combined battery-and-inverter efficiency, the nominal battery capacity is:
6.9 ÷ (0.90 × 0.92) ≈ 8.3 kWh.
The inverter must still be checked separately against the continuous and startup power results.
Begin with refrigeration, communications, efficient lighting, boiler controls, circulation pumps and medically important loads. Electric ovens, resistance heaters, electric water heaters and whole-home air-conditioning can increase the required battery and inverter size dramatically.
An essential-load panel can isolate the circuits that truly need backup and can make the system smaller, clearer and more reliable.
- Battery location, ventilation, temperature, clearances and fire requirements are product- and jurisdiction-specific.
- Backup controls and transfer equipment must prevent unintended grid energisation.
- Manufacturer usable-energy, continuous-power and surge-duration limits override generic presets.
- Cold weather, ageing and high discharge rates can reduce available capacity.
- For medical or life-safety loads, use verified runtime, alarms and redundancy rather than this calculator alone.
- U.S. Department of Energy — Solar-Plus-Storage 101
- U.S. Department of Energy — solar and storage basics
- Ready.gov — power-outage guidance
- Battery and inverter product manuals — final usable capacity, temperature range and surge limits.
- Solar Panel Size Calculator — estimate household solar generation.
- Generator Sizing Tool — compare running and starting watts for backup generation.
FAQ
Common questions about home battery capacity, inverter power and outage planning.