A base load power plant is one that provides a steady flow of power reguardless of total power demand by the grid. Typically, nuclear power plants and most coal fired power plants are considered base load because it is more efficient or safer to run at a set output instead of trying to precisely match power consumption demands. In other words, there is a minimum amount of electricity that is always used, called the base line. Massive power facilities that provide alot of the power usually take a long time to fire up or start. They also take a long time to cool back down, so it is best to leave them running all the time. Each base load power plant is aloted an amount of the base line power demand to handle. Fluctuations in power supply demand, the peak power demand or spikes in customer demand, are handled by smaller and more responsive gas and oil fired powered plants, in addition to hydro-electric.

Nuclear and coal power plants may take many hours, if not days, to achieve a steady state power output. These types of plants take a long time to heat up to operating temperature. These plants typically handle large amounts of the base line demand.

Gas and oil power plants are much faster, but have less output capacity. These types of power plants can supply power in about 30 minutes or less. They are also more expensive to operate than coal power plants. These plants typically handle the peak power demands.

Hydro-electric is the fastest to respond to increasing power demands. It can be at full power in about two to three minutes. But hydro-electric power is limited by the amount of water availible. These plants can provide both base load and peak load demands for power.