Electrical power calculator

The formulas

Electrical power in watts: P = V × I, which with Ohm's law also gives P = I² × R and P = V² ÷ R. Enter any two of voltage, current and resistance and the calculator shows the power and the missing value.

Choosing a resistor's power rating

Work out the power the resistor will dissipate, then pick a rating at least double it - a resistor run at its full rating gets very hot and drifts or fails early. Example: 220 Ω with 12 V across it dissipates 12² ÷ 220 ≈ 0.65 W, so use a 2 W part, not a 1 W one.

When you replace a burnt resistor, check whether it simply failed or was overloaded by a fault elsewhere - fitting the same value without finding the cause usually burns the new one too. Resistors and other replacement parts are in our parts catalogue.

Questions

What power rating do I need?

At least twice the calculated dissipation, and more in a hot or enclosed spot. Standard ratings are 1/8, 1/4, 1/2, 1, 2, 3, 5 W and up.

Is this the same as amplifier power?

For a steady sine wave into a speaker, yes: P = V² ÷ R with V the RMS voltage across the speaker and R its impedance. Peak and "music power" figures on spec sheets are calculated differently.