Heat rate is a term commonly used in power stations to indicate the power plant efficiency. The heat rate is the inverse of the efficiency: a lower heat rate is better.
While efficiency is a dimensionless measure (sometimes quoted in %) heat rate is typically expressed as GJ/GWh. This is because Watt-hours are more commonly used when referring to electrical energy and Joule is more commonly used when referring to thermal energy.
Heat rate in the context of power plants can be thought of as the input needed to produce one unit of output. It generally indicates the amount of fuel required to generate one unit of electricity. Performance parameters tracked for any thermal power plant like efficiency, fuel costs, plant load factor, emissions level, etc. are a function of the station heat rate and can be linked directly.
Given that heat rate and efficiency are inverse of each other, it is possible to convert from one to the other:
Most power plants have a target or design heat rate. If the actual heat rate does not match the target, the difference between the actual and target heat rate is the heat rate deviation.