At a power station, heat from fuel or a nuclear reactor boils water to make steam. This goes through turbines – machines in which steam rushes past fanlike blades and makes them spin. The turbines turn huge generators, each able to produce enough power for 20 electric trains. The steam is cooled in big towers and turns back into water, which can be reused.

Electricity leaves the power station through metal cables on tall pylons. Power is sent out at a much higher voltage than that used in homes. This is because the higher the voltage, the lower the current needed for the same power. Lower currents allow thinner cables, cutting costs, but the high voltage means that huge insulators are needed for safety.

Electricity arriving at a city is not ready to use because its voltage is much too high. Transformers at substations reduce the voltage. At a big substation, the voltage is kept quite high because the electricity still has to travel around the city and to nearby country areas. Smaller, local substations will finally reduce the voltage down to the level we use in our homes.

Big cities have complex electrical networks with miles of cable and many substations to deliver power to thousands of buildings. Some cities have overhead cables (such as Tokyo, Japan, where an earthquake could damage underground cables). In most cities, however, power travels in heavy cables that carry large electric currents under the streets.

When we switch on a cooker or heater in the home, the heat that became electricity at the power station is released again. It can even boil water, just as it did before. But there is a price to pay for the convenience of electricity. Only about a third of the heat from the fuel used to make electricity actually gets to our homes. The rest is wasted or lost on its journey.