Computers & Laptops

UPS Capacity Calculator

The UPS rating a load needs, in both VA and watts - because a unit has to satisfy both limits.

3 inputs Free, no sign-up

Your figures

Everything the UPS has to keep running - a computer and a monitor is the usual pair. Measure it with a plug-in meter if you can.

Spare capacity above the load. This is a choice rather than a rule, and the result names whichever figure it used.

From the equipment's specification. A modern supply with active correction is 0.95 or better; an older one can be 0.6. Leave blank for this page's 0.9 assumption, which the result names.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

A UPS is sold in volt-amperes and a computer's power supply is rated in watts, and they are not the same quantity. A 600 VA unit will not carry a 600 W load: at a power factor of 0.9 it is good for about 540 W, and many units are rated lower still.

Enter what the load actually is and this gives both figures, so you can check a unit against both of its ratings rather than just the one on the box.

How to use this calculator

  1. Add up everything the UPS has to keep running — the computer, the monitors, the router if you want the internet to survive too. A plug-in meter at the desk gives the real figure.
  2. Set the headroom you want above it.
  3. Enter the power factor from your equipment's specification, or leave it blank for this page's 0.9 assumption, which the result names.

The formula

Watts needed = Load x (1 + headroom / 100)
VA needed    = Watts needed / Power factor

The power factor is a property of the load, not a constant. A modern supply with active correction is 0.95 or better; an older one can be 0.6, and at 0.6 the same load needs half again as much VA capacity.

SymbolMeaningUnit
Load What has to stay running W
Headroom Spare capacity you want %
Power factor Property of the load

Worked example

A 450 W desk with 25% headroom, at a power factor of 0.9:

Watts = 450 x 1.25   = 562.5 W
VA    = 562.5 / 0.9   = 625 VA

So a unit rated at least 625 VA and at least 563 W.

Notes and limits

  • Both ratings matter. Sizing on the VA figure alone will overstate what a unit can actually run, because its watt rating is the lower limit in practice.
  • This gives a capacity, not a runtime. How long the UPS holds depends on its battery, and the UPS runtime calculator works that out.
  • A laser printer should not be on a UPS. Its warm-up draw is far above its rated figure and will trip most units.
  • The kind of UPS matters as well as the size. Some power supplies with active correction object to the stepped waveform a cheaper standby unit produces when it switches to battery.

What this calculation assumes

  • Power factor defaults to 0.9 and the result names whether it used yours or that assumption.
  • The load is one you entered. No equipment data is stored in this page.
  • This gives a capacity rather than a runtime, and does not recommend any product.

Frequently asked questions

What size UPS do I need for my PC?

Take the load in watts, add headroom, and divide by the power factor to get the VA rating. A 450 W desk with 25% headroom needs about 625 VA and 563 W - and the unit must satisfy both.

What is the difference between VA and watts?

Volt-amperes are apparent power, which is what the unit has to carry. Watts are real power, which is what does the work. W = VA x power factor, and the power factor is a property of your equipment rather than a constant.

Can I put a 600 W load on a 600 VA UPS?

No. At a typical power factor a 600 VA unit carries roughly 360-540 W, and its own watt rating will say which. This is the most common mistake in UPS sizing.

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