What is thermal efficiency a measure of?

Thermal efficiency is fundamentally a measure of how effectively a system converts the energy derived from fuel into useful work. When we consider a power system, such as a boiler or a turbine, it is vital to assess how much of the energy input—in the form of fuel—actually contributes to performing desired work or generating power, as opposed to being wasted as heat or lost in other processes.

In a power generation context, thermal efficiency is expressed as the ratio of the useful work output to the total energy input. This metric helps in evaluating and comparing different systems to understand which ones are utilizing their fuel sources optimally to produce energy or work. Therefore, the correct answer emphasizes the importance of conversion efficiency in the context of energy production, which means understanding how well a power plant or engine is performing with the energy it consumes.

In contrast, measuring steam production, fuel consumption rates, or heat losses do not directly address the essential relationship between fuel energy input and work output, which is at the core of understanding thermal efficiency.