The most efficient combination of power stations to meet demand which is variable but predictable in its variations is a combination of power plants - some to provide a constant power output 24 hours a day - baseload, and some which can be easily switched on and whose power output can be easily controlled.
Nuclear power stations are a suitable choice for the first. The fuel in a nuclear power station is a constant source of energy, and if this energy is not utilised to produce steam and drive a turbine it is wasted. Gas power stations or hydroelectric plants are a convenient choice for the second. Gas and water are easily stored and controlled and these stations are easy and fast to power up and down.