National and global energy resources: understand why some energy resources are more reliable than others

The idea

Reliability means how dependably a resource can supply energy whenever demand arises. Fossil fuels, nuclear fuel and biofuels are reliable because they are stored and can be burned or reacted on demand, whatever the weather. Renewable resources often depend on natural conditions: solar panels produce nothing at night, wind turbines need wind, and tidal schemes work only around tide cycles. Hydroelectric and geothermal supplies are steadier, but drought or local geology can still limit them. To judge reliability, ask whether output can be controlled, whether supply is continuous, and whether it can match sudden increases in demand. A useful method is to compare a gas-fired power station, switched on within minutes, with a wind farm whose output varies through the day.

What earns the marks

• Reliability describes how dependably a resource can meet demand at the time it is needed, not simply how much energy it can release in total.
• Fossil fuels, nuclear fuel and biofuels are classed as reliable because they are stored and can be used on demand, independent of weather or time of day.
• Solar, wind and tidal resources are less reliable because their output depends on conditions such as daylight, wind speed and tide cycles.
• Hydroelectric and geothermal supplies are relatively steady, but drought, reservoir levels or local geology can still reduce their reliability.
• A strong answer compares two named resources and explains how far output can be controlled or predicted, rather than listing resources alone.

Common mistakes and how to put them right

• Confusing reliability with cleanliness.

  Put it right: A resource can be reliable yet polluting, so state the criterion being judged before giving a judgement.

• Assuming all renewables are unreliable.

  Put it right: Hydroelectric and geothermal schemes can supply steady output, so avoid sweeping statements about every renewable resource.

• Treating a large total output as proof of reliability.

  Put it right: A wind farm may generate much energy across a year yet still fail to match demand on a still day, so link output to the moment of demand.

Exam tips

• Name the resource and state the condition it depends on, for example wind speed for a wind turbine, then say how that affects supply.
• Use comparative language such as more reliable than or less reliable than, because the command word understand rewards clear judgements.
• Link reliability to matching demand, mentioning that stored fuels can be used at peak times while weather-dependent resources cannot be switched on at will.