How do electricity meters measure household use?

1. All the electricity for this house comes in through one box on the wall: the electricity meter. A wheel inside it turns, and the numbers slowly climb. What is it counting?
2. Each appliance uses electricity at its own speed, called its power. A lamp only sips. A television uses about ten times as much, and a kettle more than a hundred times as much.
3. Inside an older meter, the electricity flows through coils of wire. The coils act as magnets and push on a thin metal disc. The more electricity the house is using, the faster the disc spins.
4. Tiny gears count the turns of the disc and move the number wheels. So the meter adds up two things: how much power, and for how long. The answer is energy.
5. Newer smart meters have no spinning disc. A tiny chip measures the electricity many times every second, adds it up, and sends the reading by radio. A screen at home can show what you are using right now.
6. Switch off what nobody is using and the meter slows down at once. Less energy counted means a smaller bill, and less fuel burned to make the electricity.

Tap each part. Follow the electricity in at the bottom, and see how it ends up as numbers at the top.

• Terminals. The thick wires from the street are fixed here, and so are the wires going on into the house. All the home's electricity passes through.
• Current coil. All the electricity the house is using flows through this coil of thick wire. The more that flows, the stronger a magnet it becomes.
• Voltage coil. This coil has many turns of thin wire. It is always connected, and it makes a second magnetic field that works together with the first one.
• Spinning disc. The two coils push this thin aluminum disc around without touching it. Its speed shows how much power the house is using right now.
• Brake magnet. This strong magnet gently drags on the disc. Without it the disc would keep speeding up. With it, the speed matches the power exactly.
• Gears. A screw thread on the disc's spindle turns a set of tiny gears. They slow the motion down a great deal, so many turns of the disc make one small step of the numbers.
• Register (the numbers). The number wheels show all the energy used since the meter was fitted. To find out how much you used this month, take away last month's reading.

Tap the appliances to switch them on and off. Watch the disc and the numbers. Which one uses electricity the fastest?

Experiment: The lamp is on. What happens to the disc when you switch the kettle on as well?

1. Switch on only the lamp. Watch how slowly the disc creeps.
2. Now switch on the kettle too.
3. Switch the kettle off again, and try the oven.

What we found: The disc spins much faster. A kettle uses electricity about 150 times as fast as an LED lamp, so the numbers race. What the meter counts depends on both how powerful things are and how long they stay on.

Turn on JavaScript to try it yourself.

Switch on a set of appliances that uses between 600 and 700 watts in total, then press Check.

Electricity is delivered to your home through wires, and it all passes through one box: the meter. The meter belongs to the power company. Its job is to count how much electrical energy your home has used, so the company knows how much to charge.

Two things decide how much energy something uses. The first is its power: how fast it uses electricity. Power is measured in watts. An LED lamp uses about 10 watts, a television about 100 and a kettle about 1,500. The second is time: how long it stays on. A lamp left on all night can use more energy than a kettle that boils for three minutes.

Meters count energy in kilowatt-hours. One kilowatt-hour is 1,000 watts used for one hour, enough to boil a kettle about ten times. Old meters do the counting with a spinning disc and gears. New smart meters use a tiny computer chip and send their readings by radio.

The bucket picture helps, but it has a flaw: you can pour a bucket out again, and a meter's count never goes down. It just keeps climbing for years. Each month's bill comes from how far it climbed since the last reading. Meters and their wires carry a lot of electricity, so look at yours but never touch or open it.

Key words

Power

How fast something uses electricity. It is measured in watts.

Watt

The unit of power. A kilowatt is 1,000 watts.

Energy

How much electricity has been used altogether: power multiplied by time.

Kilowatt-hour

The unit meters count in: 1,000 watts used for one hour.

Register

The row of numbers on a meter that shows the energy used so far.

Smart meter

An electronic meter that measures with a chip and sends its readings automatically.

Check your thinking

1. What happens to a meter's disc when more appliances are switched on?

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   It spins faster. More appliances use more power, and the push on the disc grows with the power.

2. Which of these uses electricity the fastest?

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   A kettle. Heating things takes a lot of power: about 1,500 watts for a kettle, against 100 for a television and 10 for an LED lamp.

3. What two things decide how much energy an appliance uses?

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   Its power and how long it is on. Energy is power multiplied by time, so both matter.

4. Last month the meter read 4,200. Today it reads 4,500. How much energy was used this month?

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   300 kilowatt-hours. The count never goes back to zero, so you subtract the old reading from the new one.

5. Which uses more energy: a 10 watt lamp left on for 10 hours, or a 1,500 watt kettle on for 3 minutes?

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   The lamp: 0.1 kilowatt-hours against 0.075. Lamp: 0.01 kilowatts times 10 hours is 0.1 kilowatt-hours. Kettle: 1.5 kilowatts times 0.05 hours is 0.075. Time matters as much as power.

6. Why does an induction meter need a brake magnet?

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   So the disc's speed stays proportional to the power. A steady push with no drag would make the disc keep accelerating. Drag that grows with speed makes it settle at a speed that matches the power.