Free calculator

Electrical power calculator

Power is how fast a part turns electrical energy into heat, light or motion, in watts (W). Pick the two values you know, type them in, and this gives the watts, the third value, and the resistor rating to buy.

Voltage across it, V

Across the part, not the whole supply.

Resistance, R

4.7k = 4,700 Ω.

Power, P = V² ÷ R

0.81 W

Voltage: 9 V
Current: 90 mA
Resistance: 100 Ω
Rating to buy: 2 W or more
Heat each minute: 48.6 J

A common ¼ W resistor would cook: this is 3.2 times its rating. Fit a 2 W part, or share the job between two or more resistors.

The rating leaves at least twice the headroom: a part run at half its rating or less stays cool and lasts.

The formulas

P = V × I

Put Ohm's law, V = I × R, into it and you get two more, for when you know the resistance:

P = I² × R and P = V²R

All three give the same answer. Use whichever fits the two values you have.

A worked example

9 V across a 100 Ω resistor:

P = 9²100 = 81100 = 0.81 W

The current is 9 ÷ 100 = 0.09 A, and P = V × I = 9 × 0.09 gives the same 0.81 W. That's over three times a common ¼ W resistor's rating, so it would cook. With twice the headroom it needs 1.62 W or more: fit a 2 W resistor, or split the job between several.

Share the heat

Two equal resistors can share a job one would struggle with, and each burns half the power. Two 200 Ω resistors side by side make 100 Ω; so do two 50 Ω end to end. At 9 V, either pair takes the same 0.81 W, but each resistor gets only about 0.4 W, so two 1 W parts will do.

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“Power and heat” is lesson 6 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 5 before it first.

Or start from the very first lesson, no account needed

The ideas behind it

Questions people ask

How do you calculate power in watts?

Multiply the voltage by the current: P = V × I. 9 V at 20 mA is 9 × 0.02 = 0.18 W. If you know the resistance instead, use P = I² × R or P = V² ÷ R.

What power rating should my resistor have?

At least twice the power it will burn. Designers keep parts at half their rating or less, so they run cool and last. A resistor burning 0.1 W is fine as ¼ W; one burning 0.2 W wants ½ W.

Why does doubling the current give four times the power?

In a resistor, doubling the current doubles the voltage across it too (V = I × R), so P = V × I goes up by 2 × 2 = 4. That's the square in P = I² × R.

What happens if a resistor goes over its power rating?

It can't shed the heat fast enough. It gets hot, discolours, drifts in value and can fail open or even burn. Fit a bigger rating, or share the job between two or more resistors.

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