Rating efficiency is complicated by the fact that there are two different ways to measure the fuel energy input:[1] Depending on which convention is used, a differences of 10% in the apparent efficiency of a gas fired plant can arise, so it is very important to know which convention, HCV or LCV (NCV or GCV) is being used.
[2] The term efficiency is a dimensionless measure (sometimes quoted in percent), and strictly heat rate is dimensionless as well, but often written as energy per energy in relevant units.
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.
[4] Given that heat rate and efficiency are inversely related to each other, it is easy to convert from one to the other.