The basic relationship

Backup time is the available battery energy divided by the load. In practice: battery capacity in ampere-hours multiplied by battery voltage gives watt-hours; multiply by the inverter efficiency and the depth of discharge you are prepared to use; divide by the load in watts.

Why the answer is optimistic

Battery capacity is rated at a specific discharge rate. At a fast discharge the delivered capacity is lower than the nameplate figure.

Capacity is rated at 25 °C. At 35 °C the available capacity is reduced and the service life falls sharply.

Capacity falls as the battery ages — a five-year-old battery does not deliver its original capacity.

The inverter is not perfectly efficient, and efficiency varies with load.

Loads rarely draw the figure on the label; measured load is usually different.

Doing it properly

Use the battery manufacturer’s discharge tables for the actual load and the target end voltage, then apply a temperature correction and an ageing allowance. That produces a figure you can rely on in the third year, not just on day one.

A practical approach

Send us the UPS model, the measured load and the autonomy you need. We will calculate the configuration from the discharge tables and state the assumptions.