Power and energy: what the watts on a label mean

Look at the label on a kettle: 2000 W. On a phone charger: 5 W. On a light bulb: 8 W. What do the watts tell you, and why do they matter when the bill arrives?

Power is how fast energy is used

You met energy earlier, measured in joules. Power is how quickly energy is used or turned into something else. The unit is the watt (W).

1 watt = 1 joule every second

A 2000 W kettle turns 2000 joules of electrical energy into heat every second. A 5 W charger uses 5 joules every second. A big number of watts means energy is being used up fast.

Power from voltage and current

For electricity there is a handy rule:

Power = voltage × current

P = V × I

A lamp with 6 V across it carrying 0.5 A has a power of 6 × 0.5 = 3 W. A 2000 W kettle on a 230 V supply draws 2000 ÷ 230 ≈ 8.7 A. That's a big current, and it is why kettles need thick cables.

Combine this with Ohm's law and you can see why more voltage or more current means more power, and why short circuits are dangerous: a huge current means a huge amount of heat, very fast.

Power adds up

Plug in several devices at once and their powers simply add. A kettle (2000 W), a heater (1000 W) and a TV (100 W) running together draw 3100 W. Try this in Watt Wise.

Energy is power × time

A 2000 W kettle sounds hungry, but it only runs for a few minutes. A 45 W fridge sounds modest, but it runs all day. To compare them we need energy:

Energy = power × time

Electricity companies use a unit that's handier than joules: the kilowatt-hour (kWh). It is the energy used by a 1000 W device running for one hour.

The fridge, with a tiny power, beats the kettle in energy, because it runs for so long. Your electricity bill is just kWh used times the price of each kWh.

Efficiency: the same light for less

An old filament bulb gives off most of its energy as heat, not light. A 60 W old bulb makes about the same amount of light as an 8 to 10 W LED. The LED turns far more of its energy into light, so you pay for less and the room stays cooler.

This is what efficient means: getting the same job done with less energy. The cheapest energy is energy you never need to use. That's the lesson of the Solar shed mission in Watt Wise.