We’re going to define power in an electric circuit as the rate tat electrical energy is used by the load (or supplied by the source). Power is then equal to the work done by the charge per unit time, or said another way, the rate that the charge changes its energy (whether it’s lost or gained).

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For a battery, the charge gains energy so it’s a positive quantity, and in the devices in the circuit (the load), energy is lost, so the change is negative. Power is measured in Watts (W), and 1 watt delivers 1 joule of energy every second.

1 Watt = 1 Joule / second

An incandescent light bulb might have a 100 Watt rating stamped on it, which means 100 joules of energy is being delivered every second to the light bulb. A 15 watt night light gets 15 joules of energy every second.

Have you ever seen an electric bill? They don’t charge you by the “watt”, but by the “kWhr” or “kWh”, which is a kilowatt-hour. (A kilowatt is 1,000 watts.) A kWhr is a unit of power per unit time, which means it’s a measure of energy. Your electric bill is a bill for energy, not power.

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