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What is reliability of supply?

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An energy source is considered reliable on this site if it can be used to generate a consistent electrical output and is available to meet predicted peaks in demand.

Every energy source has strengths and weaknesses, such as its inherent limitations on reliability of supply, which could contribute to the likelihood of an energy gap, when supply falls short of demand, and might cause interruptions to the electricity supply.

For example, generating electricity from renewable energy sources, such as wind and sunlight, can help the UK meet its carbon-reduction commitments. But these technologies produce electricity intermittently so cannot be relied on to meet peaks in demand.

Energy mix

The greater the quantity of intermittent sources of generation present in the energy mix, the greater the level of flexible backup generation required.

Consequently, to be reliable, the electricity generation mix needs a balance of generating technologies which produce stable or controllable amounts of electricity continuously, such as nuclear, coal and gas, and flexible power stations – mostly gas and hydro – which can switch on and off at short notice to meet variations in demand.

Information sources

What does it mean for the UK?

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Each of the different energy sources has individual reliability characteristics, all with important consequences for the overall reliability of the UK electricity supply.

The Energy Sources


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UK wind farms have an assumed availability at peak of 10%. Wind is intermittent. But this can potentially be addressed by storing its energy output, by providing quick-to-respond flexible backup and by distributing wind turbines across a wide geographical area.


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Solar power is an intermittent energy source. The UK does not rely on it for large-scale generation and it is largely confined to microgeneration. In the future, improved storage technologies may mean solar can play a greater role.


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UK hydroelectric power stations have an assumed availability at peak of 60%. Hydro might be a minor player in terms of total UK generating capacity, but its inherent reliability contributes to the overall reliability of the electricity supply.


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Marine energy technologies are still undergoing trials, so it is too early to judge their reliability. However, inherently they are not as subject to intermittency as some other renewable sources, such as wind and solar energy.


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UK nuclear power stations have an assumed availability at peak of 75%. Nuclear energy is one of the most reliable ways of generating electricity because nuclear power stations can deliver predictable and consistent levels of output for long periods of time.


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UK gas-fired power stations have an assumed availability at peak of 90%. Gas plays a significant role in making the UK's overall electricity supply reliable.


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UK coal-fired power stations have an assumed availability at peak of 90%. The UK relies on coal-fired power stations to generate consistent levels of electricity, but the Government's climate change commitments mean no more will be built until the technology is available to bring down their carbon emissions.


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UK oil-fired power stations have an assumed availability at peak of 80%. Oil-fired power stations can be relied on to meet peak demand, but the fuel comes at a high price and their high carbon footprint means they will not be used after 2015.