APES Energy Practice Test

Understanding how thermal efficiency is defined helps explain these numbers: it’s the fraction of the fuel’s energy that actually becomes electricity. Conventional coal-fired plants run a simple steam cycle, so they typically convert about one-third to two-fifths of the input energy into electricity, roughly 33–40%. Natural gas combined-cycle plants pair a gas turbine with a steam turbine, using the waste heat from the gas turbine to power an additional steam cycle, which raises overall efficiency to about half to sixty percent in common practice. So the statement that coal-fired plants are around 33–40% efficient and natural gas combined-cycle plants are around 50–60% efficient matches these real-world ranges. The other options propose much higher or lower values than what is typical for these technologies.