Power stations fuelled by fossil fuels or nuclear fuels are reliable sources of energy, meaning they can provide power whenever it is needed.
However, their start-up times vary according to the type of fuel used.
This list shows the type of fuel in order of start-up time:
Nuclear power stations and coal-fired power stations usually provide 'base load' electricity - they are run all 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 of fuel such as energy from the Sun and wind energy do not cost anything, however the equipment used to generate the power may be expensive to build.
Others are less reliable, including wind and solar energy.