Electrical power to the grid is the output power generated by a power plant through the use of a fuel or flow of energy. This power is output by these plants as electricity and fed to the grid via electrical transmission in order to meet society's electrical needs. These needs are constantly changing, so it is important for most of these power plants to be dispatchable in order to meet these fluctuations.
Electrical power is often contrasted with thermal power, because thermal power is often used to create electrical power. A power plant takes an input of thermal power from the burning of a fuel such as coal or natural gas. The amount of thermal power input to a power plant is most often measured in units of megawatts thermal (MWt), whereas the electrical power output is measured in megawatts electric (MWe). Due to the limits of thermodynamics, the amount of heat energy input will always be more than the useful energy received (see Carnot efficiency). Therefore the MWt value for a power plant will always be greater than the MWe value.
Electrical power is also generated by renewable energy sources such as wind turbines and solar panels. These sources are most often intermittent and non-dispatchable, meaning the electricity they generate is not readily available at all hours of the day. Without the use of energy storage, these sources cannot reliably provide electrical power to meet society's changing demands for electricity. The efficiency of these do not have thermodynamic limitations like the plants listed above, but they do have other limitations of how much electricity they can produce (see Betz limit for wind and PV cell efficiency for solar).