wattbench
Battery Tool

Battery Backup Runtime Calculator

Estimate how long a battery can support a nominated essential load from usable capacity, backup reserve and conversion efficiency.

Sources listedReviewed July 2026Our methodology

Inputs

View result
From the battery datasheet. The documented usable capacity. No default is assumed.
Capacity kept in reserve rather than used by these loads.
From the battery datasheet. The documented value for the backup operating path. No default is assumed.
Total load expected to operate at the same time.
From the battery datasheet. Or the inverter's backup specification, whichever limits. No default is assumed.

Live result

Edit inputs

Energy available

–kWh

Estimated runtime

–hours

Continuous load

–kW

Power limit

–

Enter capacity, load, efficiency and the documented power limit to check the estimate.

Key takeaways
  • Turns usable battery capacity, reserve, conversion efficiency and a continuous essential load into an indicative runtime.
  • Checks the nominated load against the backup system's continuous power limit, which is separate from stored energy.
  • Does not model start-up surge, changing loads, battery temperature or product-specific backup behaviour. Compare starting demand with the backup surge rating in the Essential Load Schedule's starting-load check, and confirm the rest against the equipment documentation.

How it works

The estimate first removes the nominated reserve from usable capacity, then applies the conversion efficiency. Dividing that available energy by the continuous load gives the indicative runtime.

Runtime is an energy check. The separate power-limit result checks whether the entered continuous load can run at once. It does not model start-up surge, changing loads, temperature or product-specific backup behaviour. To compare motor, compressor and pump starting demand with the backup surge rating, use the starting-load check in the Essential Load Schedule.

Estimates only: for guidance, not a quote or a compliance certificate. Verify against the current AS/NZS standards and a licensed design. The maths runs in your browser. Nothing is sent to Wattbench unless you sign in and deliberately save the result.

Source ledger1 source

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Frequently asked

Does this include appliance start-up surge?
No. It checks continuous load only. Motors, compressors and pumps can have a higher start-up demand. Use the starting-load check in the Essential Load Schedule to compare each load's published starting figure with the backup system's own surge rating from the battery and inverter documentation.
Why are energy and power separate?
Energy in kilowatt-hours affects how long loads can run. Power in kilowatts affects how much can run at once. A battery can have enough stored energy but still be unable to start or support a large load.
Is the result a backup design?
No. It is an estimate from the values you enter. Product limits, surge behaviour, wiring, changeover arrangements and essential-load selection require a licensed design.
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