UPS sizing calculator

VA rating scenario = watts ÷ power factor × reserve factor. Battery energy = watts × runtime hours ÷ efficiency.

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Load (watts)

Load power factor

More inputs and assumptions

Runtime wanted (minutes)

Battery-to-load efficiency (%)

Capacity reserve (%)

Answer

…

VA rating scenario = watts ÷ power factor × reserve factor. Battery energy = watts × runtime hours ÷ efficiency.

More results

Precision and calculation record

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Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.

Saved scenario

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Formula and assumptions

The efficiency and reserve are user assumptions. Energy is a first-order estimate; real UPS runtime depends on discharge rate, battery age, temperature and the manufacturer’s runtime curve. Check both watt and VA ratings and startup surges; this does not select a UPS model.

Automatic precision shows numbers to 12 significant digits. Thousands separators such as 1,000 are accepted; use a dot for decimals.

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Updated 2026-09-29

Using UPS sizing calculator

VA rating scenario = watts ÷ power factor × reserve factor. Battery energy = watts × runtime hours ÷ efficiency.

Next: Battery life calculator

Next: Electrical load calculator

A worked example

For load (watts) = 600, load power factor = 0.8, runtime wanted (minutes) = 15, battery-to-load efficiency (%) = 90, capacity reserve (%) = 20, apparent capacity scenario (va) = 900.

Before you use the result

Example results for load (watts)

These examples use Load power factor: 0.8; Runtime wanted (minutes): 15; Battery-to-load efficiency (%): 90; Capacity reserve (%): 20. They are reference calculations, not recommended settings.

Load (watts)	Apparent capacity scenario (VA)
300	450
600	900
1200	1800

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