What is electric power?

A kettle and an LED lamp can be plugged into the same kind of socket. But the kettle boils water in a few minutes, while the lamp stays only a little warm. The kettle uses energy much, much faster. How fast something uses energy is called its power.

Power is about speed

Electric devices work the same way. Power tells us how much electrical energy a device uses every second. A kettle, a heater or a hair dryer uses a lot every second. A phone charger or an LED lamp uses only a little.

Watts and kilowatts

Power is measured in watts, written W. The unit is named after James Watt, a Scottish engineer who built much better steam engines about 250 years ago.

One watt is a small amount of power. For bigger amounts, we use kilowatts (kW). One kilowatt is 1,000 watts, the same way one kilogram is 1,000 grams.

You can often find a device’s power printed on a small label, for example on the bottom of a kettle.

How much power do things use?

Device	Power
LED light bulb	5–10 W
Phone charger, while charging	5–20 W
Laptop	20–60 W
Old-style light bulb	40–100 W
TV	50–150 W
Microwave oven	1,000–1,500 W
Electric heater	1,000–2,500 W
Electric kettle	2,000–3,000 W

An LED bulb gives as much light as an old-style bulb, but it needs many times less power. The old bulb turns most of its energy into heat instead of light.

Push times flow

The power of a device depends on two things: the push and the flow. The push is the voltage, and the flow is the current. To find the power, multiply them:

$$P = V \times I$$

• $P$ is the power, in watts (W).
• $V$ is the voltage, in volts (V).
• $I$ is the current, in amperes (A).

A kettle in a 230-volt socket takes a current of about 9 amps. Its power is 230 × 9 = 2,070 watts, which is about 2,000 watts.

A phone charger gives a phone 5 volts and 2 amps. Its power is 5 × 2 = 10 watts. That is two hundred times less than the kettle.

Energy is power × time

The electricity bill for your home is mostly about energy: how much power was used, and for how long. To find the energy, multiply the power by the time:

$$E = P \times t$$

• $E$ is the energy, in kilowatt-hours (kWh).
• $P$ is the power, in kilowatts (kW).
• $t$ is the time, in hours (h).

One kilowatt-hour is the energy that a 1,000-watt device uses in one hour. A meter in every home counts the kilowatt-hours, and each one costs money.

Example. A 2,000-watt kettle boils water for 3 minutes. How much energy does it use?

In a whole hour, the kettle would use 2 kWh. But 3 minutes is only a twentieth of an hour, because 60 ÷ 3 = 20. So the kettle uses 2 ÷ 20 = 0.1 kWh.

A 10-watt LED bulb uses the same 0.1 kWh when it shines for 10 hours. The kettle uses energy fast, for a short time. The bulb uses it slowly, for a long time. In the end, they use the same amount.

Go deeper: Two more power formulas for grown-ups and the extra curious

You can combine $P = V \cdot I$ with Ohm’s law, $V = I \cdot R$. This gives two more formulas for the power turned into heat in a resistance $R$ (in ohms):

$$P = I^2 \cdot R \qquad\qquad P = \frac{V^2}{R}$$

• The first one shows that the heat made in a wire grows with the square of the current. Twice the current makes four times the heat. That is why power lines carry electricity at a very high voltage and a small current (see alternating current).
• The second one fits a kettle. Its heating part of about 26.45 Ω on 230 V gives $P = \frac{230^2}{26.45} = \frac{52{,}900}{26.45} = 2{,}000\ \text{W}$.

Go deeper: Watts, joules and kilowatt-hours for grown-ups and the extra curious

• The scientific unit of energy is the joule (J). One watt means one joule every second: $1\ \text{W} = 1\ \text{J/s}$.
• A kilowatt-hour is 1,000 watts for 3,600 seconds, which is 3,600,000 joules (3.6 MJ).
• The power on a label is the most a device uses. Many devices use less most of the time. A fridge, for example, switches its motor on and off.
• The energy a device uses never disappears. It changes form, and in the end most of it becomes heat. Read more in conservation of energy.

Go deeper: Who was James Watt? for grown-ups and the extra curious

James Watt (1736–1819) did not invent the steam engine, but he made it far better. In the steam engines of his time, the same cylinder was heated and cooled again at every stroke, which wasted a lot of coal. Watt added a separate chamber, called a condenser, where the steam was cooled. His engines used much less coal and helped to power the Industrial Revolution. In 1882, a scientist proposed naming the unit of power after him, and today every lamp and kettle is labelled in watts.

Check yourself

Which uses more energy: a 2,000-watt kettle for 3 minutes, or a 10-watt LED bulb for 10 hours?

The kettle The LED bulb Both use the same

Show the answer

Both use the same — The kettle uses 2 kW × 0.05 h = 0.1 kWh. The bulb uses 0.01 kW × 10 h = 0.1 kWh. The kettle uses energy faster, but for a much shorter time.