UPS Size Calculator

Estimate the UPS output watts and apparent power class you should look for, based on equipment load and headroom.

Result

Look for a UPS rated at least 750 W output and about 950 VA or higher based on the selected PF estimate.

Entered load

600 W

Headroom

25%

Minimum output watts

750 W

Estimated VA

~938 VA

Using PF 0.8 as an estimate

Check both the manufacturer watt rating and VA rating. A given VA class does not always have one matching watt rating.

Find UPSs that meet these requirements Listed output watts and VA only, not runtime or compatibility.

How this was calculated

Required output watts = load watts × (1 + headroom percent ÷ 100).
Estimated VA = required output watts ÷ power factor.
If you do not enter a power factor, 0.8 is used as a conservative estimate — not every load has PF 0.8.
The VA shopping floor is rounded up to the next 50 VA. Manufacturer VA and watt ratings still have to be checked separately.

Limitations

This sizes output capacity, not battery runtime. Runtime depends on the UPS battery system and the manufacturer load curve.
Startup and inrush loads may need additional margin beyond the headroom you enter.
This is conservative sizing guidance, not electrical-safety or installation advice.

Related guides

How Many VA Do I Need in a UPS?

Calculate the UPS capacity you need by totaling your equipment’s watts, converting watts to VA using power factor, and adding practical headroom. A UPS must meet both its VA and watt limits, while runtime requires a separate battery check.

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UPS Sizing Calculator: Watts, VA and Runtime

Size a UPS systematically by adding the protected load, converting between watts and VA, allowing headroom, and checking runtime separately. This guide includes a worked example and a practical UPS sizing checklist.

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