We use energy resources for transport, heating and generating electricity.

• Non-renewable: coal, oil, natural gas (the fossil fuels) and nuclear fuel. They are being used up faster than they can be replaced, and will run out.
• Renewable: wind, solar, hydroelectricity, tides, waves, geothermal and bio-fuel. Nature replaces them as fast as we use them.

Renewable does not mean perfect, and non-renewable does not mean useless. Exam questions want you to weigh both sides.

Reliable means it can produce electricity whenever it is needed.

• Fossil fuels, nuclear and geothermal: run at any time. Gas power stations (and pumped-storage hydroelectricity) can start in minutes to meet a sudden rise in demand; nuclear is slow to start.
• Tides: not constant, but predictable months ahead.
• Wind, waves: depend on the weather. Solar: the weather and daylight.

Environmental impact:

• Burning fossil fuels releases carbon dioxide (global warming); coal also releases sulfur dioxide (acid rain).
• Nuclear: no carbon dioxide while running, but radioactive waste and a risk of accidents.
• Renewables: no fuel to burn, but they take up land or change habitats (dams flood valleys, barrages change estuaries), and some people object to how wind turbines look.

Questions give you a chart or table of where electricity comes from (the "generation mix").

1. A country: gas 38%, wind 24%, nuclear 16%, solar 6%, biomass 8%, coal 2%, hydroelectricity ?
2. other sources add up to 38 + 24 + 16 + 6 + 8 + 2 = 94%
3. hydroelectricity = 100 − 94 = 6%
4. renewable = 24 + 6 + 8 + 6 = 44%
5. if the total is 300 TWh, gas gave 38 ÷ 100 × 300 = 114 TWh

A fall from 30% to 20% is a fall of 10 percentage points. If the total also changed, work out the energy in each year before you compare.

For "evaluate" questions (often 6 marks): use the numbers given, cover both sides (reliability, cost, emissions, land, build time), then make a judgement and say why.