Backup power planning

Battery Runtime Calculator

Translate battery watt-hours and real running loads into a conservative runtime estimate, then verify the exact setup before an outage.

These planning tools need JavaScript to calculate results. You can still use the linked guides and safety sources on this page.

Estimate battery runtime from watt-hours

Use the nameplate energy and continuous-output ratings for the exact system. Then add conservative reserve and conversion-loss assumptions.

Battery system ratings

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

Nameplate capacity (Wh)

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

Continuous output (W)

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.

Refrigerator150 W example running loadLED lights60 W example running loadInternet / phone charging45 W example running loadMedical device120 W example running loadSump pump800 W example running load

Custom running load (W)

Add other simultaneous running watts.

Planning assumptions

Use the manual if it provides a usable-capacity or efficiency figure. Conservative inputs reduce overconfidence.

Capacity held in reserve (%)

Conversion efficiency (%)

Use the result well

Check energy, power, and the real setup

Runtime is an energy estimate. The system still needs enough continuous and surge power, safe placement, compatible charging, and a real test with the exact devices.

Step 1
Copy watt-hour capacity and continuous AC output from the exact system manual.
Step 2
Measure essential running loads and verify startup demand for motors, compressors, and pumps.
Step 3
Use conservative reserve and efficiency inputs, then perform a supervised discharge test before relying on the result.

Questions about this estimate

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.