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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
Use the product's documented usable capacity.
Capacity kept in reserve rather than used by these loads.
Use the documented value for the relevant operating path.
Total load expected to operate at the same time.
From the battery or inverter backup specification.

Live result

Edit inputs

Energy available

8.1kWh

Estimated runtime

8.1hours

Continuous load

1.0kW

Power limit

Within

The continuous load is within the entered power limit. Start-up surge and product-specific backup rules still need checking.

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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. Confirm those 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.

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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Good to know

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, which must be checked against 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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