Thermal power describes how fast heat is produced. For most energy systems such as a gasoline engine, thermal power is how fast fuel is converted into heat. These heat engines create this heat to achieve useful work. Most commonly thermal power refers to the heat input to a boiler in a power plant in order to generate electricity. In other contexts, it can be a measure of the output—such as the radiant heat given off by the Sun.
For power plants, the thermal power input is measured in megawatts thermal (MWt). However, the output, which is usually supplying electric power to the grid is measured in megawatts electric (MWe). Since not all of the input heat can be converted entirely into electricity (see Carnot efficiency), the MWt value will always be larger the the MWe value. The comparison gives a power plant its thermal efficiency, which is a measure of how much useful work it can achieve for the amount of fuel it had to burn. Most older power plants are around 33% efficiency, so the input thermal power is 3x more than the output electrical power.
Not all power plants require thermal power in order to generate electricity. Plants like hydroelectric facilities, wind turbines, or photovoltaic cells use other forms of power from different energy flows to create electricity. Therefore, thermal power is only used for thermodynamic systems. Although the thermodynamic limits do not apply for these plants, such as the second law of thermodynamics and Carnot efficiency, they do have other factors that limit their efficiency.