A base load generation unit is one that provides a steady flow of power regardless of total power demand by the grid. This unit runs all seasons except during the time when repairs or scheduled maintenance occur. Base-load plants usually run on low-cost fuels such as coal and are massive enough to provide a majority of the power used by a grid. Therefore, these plants have high capital costs to build but low operating costs to run. In contrast, peak-load units (also known as peakers) are power plants that generally run only when there is a high demand, known as peak demand, for electricity. The time a peaker operates may be several hours a day to as little as a few hours per year. If a peaker is only going to be run for a short and variable time, it does not make economic sense to make it as efficient as a base-load power plant. Therefore, peak-load systems tend to have low capital costs (so it is OK if it lying idle for most of the year) but high operating costs (but then, you don’t run it that often).