Defining and Calculating Electrical Power

What is electrical power?

Electrical power P represents the amount of energy transferred or transformed by an electrical device each second. The more powerful a device is, the faster it consumes (or supplies) energy. Power is measured in watts, denoted W, in honor of James Watt.

The fundamental relationship

For a two-terminal device carrying a current, electrical power is calculated as:

P = U x I

where:

• P is the power in watts (W)
• U is the voltage across it in volts (V)
• I is the current flowing through it in amperes (A)

The units check out: 1 W = 1 V x 1 A.

Concrete example

An electric heater is connected to a voltage U = 230 V. It carries a current of intensity I = 4 A. Its power is:

P = U x I = 230 x 4 = 920 W

Orders of magnitude

Device	Typical power
Phone charger	5 to 20 W
LED bulb	5 to 10 W
Laptop	50 to 100 W
Electric heater	1000 to 2000 W
Oven	2000 to 3000 W

Common pitfalls

Do not confuse the voltage U (in volts) with the current I (in amperes): these are two different quantities, and swapping them in the formula gives a wrong result in most practical cases. Also watch the units: a device sometimes states its power in kilowatts (kW). You then need to convert: 1 kW = 1000 W before comparing or adding powers together.