Computers & Laptops

PSU Efficiency Calculator

Measured power supply efficiency from the wall reading and the DC output, with the losses in watts.

2 inputs Free, no sign-up

Your figures

What the supply draws from the wall, from a plug-in energy meter.

What the components on the other side are drawing. Software monitoring or a bench measurement will give it.

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 power supply converts AC from the wall into DC for the components, and loses some of it as heat on the way. Efficiency is the ratio between the two, and measuring it takes a reading at each end.

The difference is not lost quietly: it becomes heat inside the case, which the fans then have to move out.

How to use this calculator

  1. Measure the AC input with a plug-in energy meter between the wall and the machine.
  2. Get the DC output figure — from bench equipment, or from software that reports package and board power if that is the best available.
  3. Enter both. The AC figure must be the larger of the two.

The formula

Efficiency = DC output / AC input x 100
Lost       = AC input - DC output

A result above 100% means the readings are wrong rather than that the supply is remarkable, so the page says so instead of printing it.

SymbolMeaningUnit
AC input Drawn from the wall W
DC output Delivered to components W

Worked example

500 W at the wall, 440 W reaching the components:

Efficiency = 440 / 500 x 100 = 88%
Lost       = 500 - 440        = 60 W

Sixty watts of heat, all of it inside the case.

Notes and limits

  • This is the efficiency at the load you measured, not a rating for the unit. Efficiency varies across the load range — typically poorest below about 20% of capacity and best somewhere near the middle.
  • Certification marks describe efficiency at set load points under set conditions. A plug-in meter at home will not match them exactly and is not meant to.
  • The losses cost money as well as making heat: 60 W of loss for eight hours a day is roughly 175 kWh a year.
  • A cheap meter has its own error, often a few per cent, and reads low power factor loads badly. Treat a single reading as indicative.

What this calculation assumes

  • Both readings are yours, taken at the same load.
  • The result describes one operating point rather than the supply as a whole.
  • A result above 100% is refused, because it means the two readings do not belong together.

Frequently asked questions

How do I measure PSU efficiency?

Measure what the machine draws from the wall with a plug-in meter, find what the components are actually consuming, and divide the second by the first. Both readings have to be taken at the same moment under the same load.

What is a good efficiency figure?

Modern supplies are commonly in the high eighties to low nineties in the middle of their range. This page reports what you measured rather than grading it, because the honest comparison is against the same unit at the same load point.

Why is my figure lower than the certification claims?

Certifications are measured at specific load percentages, at a specified input voltage and temperature, with laboratory equipment. A house meter at an arbitrary load will read differently, and usually lower.

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