Heat rate (efficiency)

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.

${\text{Heat Rate}}={\frac {\text{Thermal Energy In}}{\text{Electrical Energy Out}}}$

While efficiency is a dimensionless measure (sometimes quoted in %) heat rate is typically expressed as Btu/kWh. This is because Watt-hours are more commonly used when referring to electrical energy and Btu 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 inversely related to each other, it is possible to convert from one to the other:

• A 100% efficiency implies equal input and output: for 1 kWh of output, the input must be 1 kWh. This thermal energy input of 1 kWh = 3.6 MJ = 3,412 Btu (British thermal units)
• Therefore, the heat rate of a 100% efficient plant is simply 3.6 MJ/kWh, or 3,412 Btu/kWh
• The U.S. Energy Information Administration gives a general explanation for how to translate a heat rate value into a power plant's efficiency value. To express the efficiency of a generator or power plant as a percentage, you can divide the equivalent Btu content of a kWh of electricity (3,412 Btu) by the heat rate. For example, if the heat rate is 10,500 Btu, the efficiency is 32.5% (since 3,412 Btu / 10,500 Btu = 32.5%). If the heat rate is 7,500 Btu, the efficiency is 45.5% (since 3,412 Btu / 7,500 Btu = 45.5%). The higher the heat rate (i.e. the more energy input that is required to produce 1 kWh), the lower the efficiency of the power plant.

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.