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Why does a lower heat rate indicate a more efficient steam turbine system?

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In the context of mechanical engineering, the heat rate is defined as the amount of heat energy required to produce a specific unit of electrical power output, typically measured in Btus per kilowatt-hour.1

A lower heat rate indicates a more efficient steam turbine system because it signifies that the system requires less thermal energy input to generate the same amount of electricity. Essentially, as the heat rate decreases, the system demonstrates an improved ability to convert the thermal energy of steam into useful work while minimizing losses from mechanical friction, bearings, exhaust, and the generator.1

Conversely, higher heat rates reflect lower efficiency, which often occurs during part-load operation or when the system fails to effectively capture the energy potential of the steam. Engineers strive to lower the heat rate by optimizing parameters such as steam pressure and temperature, though these improvements must be balanced against the increased costs of the equipment required to maintain higher operating conditions.1