Power Plant Heat Rate

Heat rate is efficiency turned upside down. A power station burns fuel and sends out electricity, and $HR=Q_{in}/E_{out}$ asks how much fuel heat went in for each unit of electricity that came out. Efficiency puts the electricity on top and the heat underneath, so heat rate is its reciprocal, which is why a lower number is better: the same electricity for less fuel. Both quantities must cover the same stretch of time. $Q_{in}$ is the fuel burned multiplied by its heating value, and $E_{out}$ is the electricity delivered to the grid in that time.

The scale does not start at zero, and that is the most useful thing to know about it. A plant that wasted nothing would still need heat: 3.6 MJ for each kilowatt hour of electricity, which is 3,412 BTU/kWh, or exactly 1 kWh/kWh at 100 % efficiency. Every real plant sits above that floor, and the distance above it is heat that never became electricity. A modern combined-cycle gas plant runs near 7,000 BTU/kWh (about 49 %) and a coal plant near 10,000 (about 34 %). Read a heat rate against 3,412, not against zero.

Heat rate is quoted in more than one unit system, and the page converts between them. BTU per kWh, kJ per kWh and MMBTU per MWh all describe the same ratio. By the definition of the units, 1 MMBTU/MWh is 10⁶ BTU over 1,000 kWh, which is exactly 1,000 BTU/kWh, so 7.5 MMBTU per MWh is 7,500 BTU/kWh. The conversion to kilojoules uses the international-table BTU, 1,055.05585262 J. That part is pure unit arithmetic; the plant enters only through the ratio of heat in to electricity out.

The page's example is 9,500 BTU of heat for 1 kWh of electricity. Entered in BTU and kWh it is a heat rate of 9,500 BTU/kWh; asked for in kJ/kWh it is 10,023.03, because 9,500 × 1.05505585262 kJ is 10,023.0306 kJ. Same plant, different unit. The rearranged form $Q_{in}=HR\times E_{out}$ turns a heat rate into a fuel requirement: 100 MWh at 7,000 BTU/kWh needs 700 MMBTU of heat. Heat rate says nothing about how many hours the plant ran or how much of its rated output it delivered; for that, set it beside the capacity factor.