heat until needed to make steam for power generation. Base load power plants An economical electrical supply system will also include base load power plants. These generating units will emphasize low incremental fuel cost, but may use a higher capital investment to improve efficiency. For example, a peaking plant might use only a gas turbine, while a base load plant might also add a steam "bottom cycle" to improve the overall plant fuel consumption per unit of electricity produced. Nuclear and coal burning plants generally operate continuously, stopping only for maintenance or unexpected outages. The low incremental fuel cost of nuclear power plants compared with high capital cost makes it most economic for them to be used for base load supply. Hydroelectric plants with few restrictions on water supply may be used for base load as their incremental fuel cost is zero. Since a steam cycle power plant may take hours to go from cold standby to full rating, they are not usually used to provide peak load service. Intermediate load following power plants such as hydroelectric operate between these extremes, curtailing their output on nights and weekends when demand is low. Base load and intermediate plants are used preferentially to meet electrical demand because the lower efficiencies of peaker plants make them more expensive to operate. See also Geothermal power List of energy storage projects Smart grid Vehicle-to-grid Turbine inlet air cooling Load following power plant References Power station technology