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Useful places to start

What should my household do first during an outage? A private, fixed-choice plan for the household systems an outage can disrupt. How much water should I store? A drinking and cooking baseline, confirmed extra water, container capacity, and filled weight. Is my fridge or freezer food still usable? A refrigerator/freezer timeline that hands off from elapsed time to temperature evidence. How long will my battery last? Runtime from constant watts or measured 24-hour energy, with a separate running-output check.

Calculators Backup power planning

How long will my battery last?

Estimate runtime from simultaneous running watts or measured 24-hour energy, with a separate check against continuous output.

About 3 minutes You’ll need: battery capacity, running watts or measured daily energy, and reserve

Battery runtime calculator for household backup power planning

Saves on this devicePrivacy & options

Calculations run in this browser; inputs stay out of analytics. A draft saves here after your first change unless you turn saving off. You choose separately whether to save a result to My Household Plan or copy a permitted share link.

Quick outage scenarios

Use a scenario only as a starting point. Every listed watt value is an example until you replace it with the exact appliance or device data.

Battery system ratings

Capacity in watt-hours is energy. Continuous watts is the maximum running power the system can deliver.

Use the manufacturer nameplate figure, before reserve and conversion losses.

Use continuous AC output, not a short surge or peak rating.

Loads to support

These are example running watts. Measure the exact devices where possible and check motor startup demand separately.

This adds watts to checked example loads. Deselect example loads to replace them with your actual simultaneous running-watt total here.

Energy-use basis

Choose whether runtime uses continuous running watts or the measured 24-hour energy for this same set of loads. Continuous output is still checked against running watts.

Constant mode assumes the running load continues; measured mode uses the 24-hour AC total below to estimate average energy load.

Planning assumptions

Set the outage window first. The reserve and conversion assumptions stay visible below and can be adjusted when you have better manual or measured values.

Use 1–72 hours. This sizes a target energy window using the same load and loss assumptions as the runtime estimate.

Battery reserve & conversion assumptions20% reserve · 85% efficiency

Use the exact manual or measured values when available. Conservative assumptions reduce overconfidence; do not apply a reserve or conversion loss twice.

A measured-energy example

If the same appliances use 900 Wh of AC energy over 24 hours, that is an average energy load of 37.5 W. Their simultaneous running demand can still be 150 W or more; this illustrative running figure is checked against continuous output, while motor startup demand remains a separate question. The measured energy total describes use and does not have reserve or conversion losses applied in advance.

For an illustrative 1,000 Wh battery, a 20% reserve and 85% conversion-efficiency assumption leave 680 Wh of estimated usable AC energy. At 37.5 W average use, that gives about 18.1 hours (680 ÷ 37.5). These percentages are example assumptions, not ratings for a particular battery. Temperature, standby draw, changing use, and manufacturer limits can change the result.

DOE's storage overview explains energy versus power capacity and conversion losses; it does not prescribe the reserve or efficiency used in this example.

Check energy, target duration, power, and the real setup

Use either a constant running-load estimate or measured total AC energy for the selected devices over 24 hours. The system still needs enough continuous and surge power, safe placement, compatible charging, and a real test with the exact devices.

  1. Copy watt-hour capacity and continuous AC output from the exact system manual.
  2. Use a quick scenario only as a starting point. For measured mode, enter one 24-hour AC energy total for the same selected and custom loads; convert kWh to Wh by multiplying by 1,000.
  3. Choose a target outage window, compare simultaneous running watts with continuous output, verify startup demand separately, and perform a supervised discharge test before relying on the result.

Common questions

How do I calculate battery runtime from watt-hours?

Estimate usable AC energy as nameplate watt-hours × (1 − reserve as a decimal) × conversion efficiency as a decimal. Divide by constant running watts for hours of runtime. For measured-energy mode, divide usable energy by the measured 24-hour AC watt-hours divided by 24. These are planning estimates; continuous output and motor startup need separate checks.

What is the difference between watts and watt-hours?

Watts measure power at a moment. Watt-hours measure stored energy. A battery must have enough continuous and surge power for the devices and enough usable energy for the planned runtime.

Why is usable energy lower than the nameplate capacity?

The estimate holds back the reserve you choose and applies a conversion-efficiency assumption. Actual usable energy also varies by battery limits, inverter, temperature, age, settings, and load.

Will the calculator confirm that a refrigerator can start?

No. Motors and compressors can draw more power while starting than while running. Check the exact appliance demand and the battery system's surge rating, duration, and manufacturer limits; this remains unresolved even when you enter measured daily energy.

Can I use this to estimate what battery size I need?

Yes, as a planning estimate. Choose a constant running-load estimate or enter measured total AC energy for the same selected loads over 24 hours. The tool applies your reserve and conversion-efficiency assumptions once. Startup power, temperature, charging, and manufacturer limits still need separate verification.

Does a runtime estimate show whether refrigerated food is safe?

No. Battery runtime does not establish food temperature or safe storage time. Use the Food Safety Outage Calculator linked under Further reading for the separate refrigerator and freezer timeline question.