The power for a particular grid generally has two components, that which is always on and that which fluctuates through the day. To give you an example, an alum

The power for a particular grid generally has two components, that which is always on and that which fluctuates through the day. To give you an example, an aluminum smelter will always use a fixed (and very large) quantity of electricity, day or night, rain or shine. As it is impractical to store electricity anywhere near the scale required to power these types of buildings, you need a power plant that can provide cheaply a continuous supply of power. These are known as your base load power plants. To show you that the storage of large quantities of is impractical by example, think of the last time you visited a data center, and remember the rows upon rows of of UPS batteries required to run one datacenter for a few minutes. It stands to reason that enough batteries to power the datacenter for an extended period of time (ie. days), would be utterly impractical. This is why datacenters have generators, ie. the batteries can't power the datanceter indefinitely and when the batteries run out, it needs to produce the power to power the datacenter. [1] http://en.wikipedia.org/wiki/Base_load_power_plant http://en.wikipedia.org/wiki/Base_load_power_plant