Power: how much work the circuit is doing

Power is volts times amps: P = V × I, in watts.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

A breaker is chosen by amps, but a heater is sold by watts and a customer asks in watts. Converting between the two is the daily arithmetic of the trade, and the day two heaters share one kitchen circuit is the day it decides whether the breaker holds.

How to think about it

Ask what the load is rated in. If it is watts and you know the volts, current is watts divided by volts. If you know amps and volts, watts is the product. Write the unit on the answer and see whether it makes sense for the circuit it is on.

Worked example

P = V × I

Watts equals volts times amps.

1500 W = 120 V × I

A 1500-watt space heater on a 120-volt circuit.

I = 1500 / 120 = 12.5 A

Twelve and a half amps, which is most of a 15 A circuit.

P = 240 V × 20 A = 4800 W

The other way round: a 240 V load drawing 20 A is 4800 W of heat.

Your turn

A 240 V heater draws 5 A. Write the line that gives its power.

P = 240 ×  = 1200 W

The trap

Forgetting that the same watts draw half the amps at twice the volts. A 240 V load is not twice as heavy on the wire; it is half as heavy.

Practise power, in watts on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.