Battery Backup Runtime Calculator
How long will your battery or portable power station run a refrigerator, lights and Wi-Fi? Enter battery capacity, appliance watts and daily operating hours to estimate backup runtime and compare 8–48-hour capacity needs. Check inverter output and motor startup requirements separately.
JHOTAS-LAB / Outage planning
Blackout Survival Calculator
How Long Can Your Home Survive a Power Outage? Estimate battery runtime for the appliances you choose, check output limits and compare essential-load scenarios. For fuel generators, use a manufacturer runtime reference.
Your backup estimate
- Never run a fuel generator indoors, in a garage or basement. CDC recommends keeping it outdoors at least 20 feet from doors, windows and vents. Use working CO alarms.
- Connecting a generator to house wiring requires appropriate transfer equipment and professional installation. Never backfeed through an outlet.
- Verify starting power and surge duration with the equipment manufacturer.
- 240 V appliances require a backup source with the appropriate 240 V output.
- Do not rely on this result alone for life-support or medical equipment. Arrange a device-specific backup and emergency plan.
How backup runtime is estimated
Average demand = sum of appliance watts × equivalent operating hours per day ÷ 24. This accounts for your assumed cycling rather than running every appliance continuously. All-selected running watts is a conservative simultaneous load check, not an average and not motor starting power.
Usable AC energy = nominal Wh × usable fraction × inverter efficiency × derating factor. Battery runtime = usable AC Wh ÷ average W. For 2,000 Wh × 0.90 × 0.90 ÷ 200 W, the estimate is 8.1 hours with a 100% derating factor. Required nominal battery energy = average W × target hours ÷ the same factors.
kWh × 1,000 = Wh. Ah × battery voltage = Wh. Nominal capacity and usable capacity are different: if entering already usable capacity, set the usable percentage to 100% to avoid deducting it twice. If the quoted capacity is already usable AC energy, avoid applying the same conversion losses again.
Why does a refrigerator use hours per day?
The compressor cycles. Enter an equivalent daily run time for your conditions or use measured daily Wh to estimate average power. Heat, door openings and startup affect real behavior.
Does enough energy mean every appliance will start?
No. Continuous output, starting power, surge duration, waveform and voltage compatibility must also be checked.
Can this predict generator fuel runtime?
No fuel curve is assumed. Generator mode shows your manufacturer runtime at its specified test load and fuel quantity, together with your selected loads. It does not scale that time to a different load.
Can I use it to size medical backup?
Use it only as an educational starting point. Manufacturer guidance, your care team and a redundant emergency plan are necessary.
Continue reading
- Can a home battery run central air?
- Generator sizing for essential appliances
- Battery energy losses and efficiency
Official References
Build a realistic refrigerator, lights and Wi-Fi backup plan
Use the battery backup runtime calculator above to compare stored battery energy with the average demand of the loads you select. Check two separate questions: how long the energy might last, and whether the inverter can operate and start the equipment.
Choose the right capacity and output inputs
kW measures power; kWh measures energy. A 2 kWh battery and a 2 kW inverter describe different limits. In the tool, choose Wh or kWh for battery energy, or Ah with the correct nominal battery voltage. Enter continuous and surge output in watts, and confirm how long the manufacturer’s surge rating is available.
Use the usable-capacity percentage for the portion available during your outage plan. Apply inverter efficiency for conversion losses and the derating factor for known age or temperature effects. The factor 100% means no extra derating. Avoid deducting the same reserve or conversion loss twice if the published capacity already accounts for it.
Worked example using the calculator defaults
| Selected load | Running watts | Equivalent hours/day | Daily energy |
|---|---|---|---|
| Refrigerator | 150 W | 8 | 1,200 Wh |
| Wi-Fi router | 12 W | 24 | 288 Wh |
| LED lights | 30 W | 5 | 150 Wh |
Total energy is 1,638 Wh/day, or an average of 68.25 W. A nominal 2,000 Wh battery at 90% usable capacity, 90% inverter efficiency and 100% derating provides 1,620 Wh of modeled AC energy. The estimate is 1,620 ÷ 68.25 = 23.74 hours.
All selected loads running together total 192 W, which differs from the average demand. These are examples, not measured refrigerator specifications. Eight equivalent operating hours expresses compressor cycling; it does not mean leaving food unpowered for the other sixteen hours.
8, 12, 24 and 48-hour scenarios
| Target | AC energy needed | Nominal battery capacity |
|---|---|---|
| 8 hours | 0.546 kWh | 0.67 kWh |
| 12 hours | 0.819 kWh | 1.01 kWh |
| 24 hours | 1.638 kWh | 2.02 kWh |
| 48 hours | 3.276 kWh | 4.04 kWh |
Round displayed values are planning estimates, not purchase specifications. Leave a practical reserve appropriate to the equipment and outage risk. For larger systems, read how long a solar battery can power a house.
Starting a motor is a separate check
A refrigerator compressor, well pump or AC compressor can need substantially more current during startup than while running. The tool does not infer starting demand from the running watts. Check voltage, peak demand, surge duration and manufacturer compatibility, including the other loads operating when the motor starts. A result within the continuous rating cannot confirm startup.
Why real runtime can be shorter
The model spreads a daily schedule into average demand. A short outage during heavy use may differ markedly. Heat, frequent refrigerator door openings, battery aging, cold-weather limits, inverter idle consumption and cut-off settings can change actual runtime. Solar recharge depends on weather and an outage-capable system; this battery estimate does not automatically add it. See battery energy losses and efficiency for the distinction between conversion and round-trip losses.
More backup questions
Does turning off the lights guarantee a full day of backup?
No. It lowers the modeled average load, but the actual operating schedule, battery condition and inverter consumption still matter. Recalculate using measured loads and keep an appropriate reserve.
Can I convert a fuel tank directly into battery kWh?
Not with this calculator. Generator runtime depends on its fuel-consumption curve and load. Select generator mode only with a manufacturer runtime reference at a stated load.
Safety and limits
Use appropriate transfer equipment for house wiring and never backfeed through an outlet. Operate fuel generators outdoors more than 20 feet from doors, windows and vents, following CDC guidance and manufacturer instructions; use working CO alarms. This tool does not validate electrical installation or medical-device backup. Arrange a device-specific emergency plan for critical equipment. Browse all free calculators.
Sources & Official References
- EIA: measuring electricity — power and energy units.
- DOE: solar energy and storage basics — storage and conversion context.
- CDC: carbon monoxide poisoning prevention — generator placement and CO protection.
Examples and references reviewed October 16, 2026. Results are estimates, not a runtime or safety guarantee.