Mobile & Electronics

Electrical Power Calculator

Watts from volts and amps, in whatever units you have them - and the missing one when you have the other two.

7 inputs Free, no sign-up

Your figures

From the label on the supply, or measured.

What is flowing, or what the supply is rated to deliver.

Leave blank to calculate it. Fill it in instead of the voltage or the current to work that one out.

Leave at 1 for DC or a purely resistive load such as a heater. A motor or a switching supply is lower, and the figure is on its rating plate.

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

Power is voltage times current. That single relationship connects almost everything on a charger label, a rating plate or a battery specification, and most of the confusion about electricity comes from trying to use one of the three numbers on its own.

Enter any two and this works out the third. Enter a voltage by itself and it will tell you why that is not enough — which is the useful answer, not an evasion.

How to use this calculator

  1. Enter the two values you know, each with the unit it is written in. Milliamps and amps are both accepted, and mixing them up is the commonest way to get an answer out by a thousand.
  2. Leave the third field blank. That is the one you get back.
  3. For AC with a motor or a switching supply, set the power factor from its rating plate. Leave it at 1 for DC and for resistive loads.

The formula

Power   = Voltage x Current      P = V x I
Current = Power / Voltage        I = P / V
Voltage = Power / Current        V = P / I

On AC, volts times amps gives apparent power, measured in VA. The real power in watts is that figure times the power factor, and for anything with a motor or a switching supply the two differ. A power factor of 1 makes them the same, which is correct for DC and for purely resistive loads such as a heater or an incandescent bulb.

SymbolMeaningUnit
P Power W
V Voltage V
I Current A

Worked example

A phone charger labelled 5 V, 3 A:

P = 5 x 3 = 15 W

A 1,000 W appliance on a 230 V supply:

I = 1000 / 230 = 4.35 A

The second is the calculation that matters when you are wondering whether an extension lead will cope.

Notes and limits

  • A rating is a maximum, not a measurement. A 5 V 3 A charger can deliver 15 W; a phone plugged into it draws whatever it is willing to draw, which is often much less.
  • Milliamps and amps are a factor of a thousand apart. A 500 mA supply is 0.5 A, and entering it as 500 A would produce a very confident wrong answer — hence the unit selector.
  • This is the DC relationship, and the AC one with a power factor applied. It does not cover three-phase supplies, where an additional root-three factor applies.
  • Voltage alone never gives power. If that is what you have, the figure you need next is the current.

What this calculation assumes

  • Values are taken as entered, in the units selected beside each one.
  • A power factor of 1 treats the circuit as DC or as a purely resistive AC load.
  • A rating is a maximum; a device rarely draws its full rating continuously.

Frequently asked questions

How do I calculate watts from volts and amps?

Multiply them. Volts times amps gives watts, so a 5 V supply delivering 3 A is 15 W. On an AC circuit with a motor or a switching supply, multiply by the power factor as well.

Can I work out watts from volts alone?

No. Voltage describes the push, not how much is flowing - a 5 V charger can be 5 W or 100 W depending on the current it delivers. You need the current, or the power, as a second figure.

What is a power factor and do I need one?

It is the fraction of apparent power that does real work, between 0 and 1. For DC and for resistive loads such as a heater it is 1 and can be ignored. Motors, transformers and cheap switching supplies are lower, and the figure comes from the rating plate.

Why does my calculation not match the label on my device?

Rating plates usually give a maximum. A device drawing less than its rating is normal. A device apparently drawing more is worth investigating - check whether the label is quoting VA rather than W.

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