Why do some power plants never switch off while others run a few hours a year?

Think of a restaurant. Some ovens are lit all day because the kitchen always needs them, and a few special ones are fired up only for the Friday rush. A power grid works the same way. Demand for electricity rises and falls, but it never drops to zero, and the minimum level over a span of time such as a week is called the base load. Power plants that cannot change their output quickly, such as some large coal or nuclear plants, are called baseload plants. They run almost all the time. Peaking plants, or peakers, run only when demand is high.

Who runs when is mostly a matter of cost. Most grids share the load in merit order, cheapest first, so cheap producers run flat out almost all the time and the more expensive ones only when necessary. Nuclear and coal plants traditionally had high fixed costs but low marginal costs, so you want them running constantly. Peaker plants are generally gas turbines, with low fixed costs and a high price per unit of power. A peaker may run many hours a day or only a few hours per year, and because it supplies power only occasionally, its electricity commands a much higher price per kilowatt hour than base load power.

When is the peak? In temperate climates it often comes in the evening, when people get home and run their appliances. In hot climates, it is usually late afternoon, when air conditioning load is high.

This tidy picture is changing. There is no technical rule that the base load must come from unvarying plants, and in 2015 National Grid's chief executive Steve Holliday and others called baseload outdated.