The heat energy for power stations comes from a variety of sources. Some of these are non-renewable, such as oil, coal, natural gas and nuclear fuels. Others are renewable, such as wood and straw.

The cost of electricity depends on the power rating of the appliance used, how long it is used for, and the price of a unit of electricity.

Voltage, current and power

Power

Power is a measure of how quickly energy is transferred. You can work out power using this equation:

power (watt, W) = voltage (volt, V) × current (ampere, A)

The unit

The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1kWh.

The equation below shows the relationship between energy transferred, power and time:

energy transferred (kWh) = power (kW) × time (h)

Note that power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you must divide by 1000.

For example, 2000W = 2000 ÷ 1000 = 2kW.

Also note that time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3600.

For example, 1800s = 1800 ÷ 3600 = 0.5 hours.

The cost

Electricity meters measure the number of units of electricity (the number of kWh) used in a home or other building. The more units used, the greater the cost. The cost of the electricity used is calculated using this equation:

total cost = number of units × cost per unit

For example, if 5 units of electricity are used at a cost of 8p per unit, the total cost will be 5 × 8 = 40p.

Conventional power stations burn fossil fuels (oil, coal or natural gas). Renewablebiomass fuels may also be used, including wood, straw or manure (which generates methane gas as it rots).

Nuclear reactors use the heat from nuclear reactions in the nuclear fuel to boil water. Just as in conventional power stations, the steam from the boiling water makes a turbine spin, which in turn makes the generator turn.

Unlike conventional power stations, however, nuclear power stations do not release carbon dioxide, so they do not contribute to global warming while in use.

-

Uranium and uranium oxide are often used as nuclear fuels. Their supplies are limited, so they are a non-renewable resource.

Ionising radiation in the reactor itself is prevented from escaping by steel and thick concrete walls. This is important because radiation can cause cancer. The waste from nuclear power stations is radioactive, so it can be harmful, too.

Plutonium is a waste product from nuclear reactors. It can be used to make nuclear bombs.

Power stations fuelled by fossil fuels or nuclear fuels are reliable sources of energy. This means they can provide power whenever it is needed. However, their start-up times vary according to the type of fuel used.

1. gas-fired station (shortest start-up time)
2. oil-fired station
3. coal-fired station
4. nuclear power station (longest start-up time)

Nuclear power stations and coal-fired power stations usually provide ‘base load’ electricity - they are run all of the time because they take the longest time to start up. Oil-fired and gas-fired power stations are often used to provide extra electricity at peak times, because they take the least time to start up.

The fuel for nuclear power stations is relatively cheap, but the power stations themselves are expensive to build. It is also very expensive to dismantle old nuclear power stations, and to store their radioactive waste, which is a dangerous health hazard.

Renewable resources

Renewable fuel resources do not cost anything, but the equipment used to generate the power may be expensive to build. Certain resources are reliable, including tidal barrages and hydroelectric power. Others are less reliable, including wind and solar energy.