Efficiency Definition

A coal plant converts 38% of its fuel energy into electricity. A solar panel converts 22% of sunlight into electricity. It sounds like coal is "more efficient." But the coal plant's ceiling is set by thermodynamics at about 65%, while the solar panel's ceiling is set by semiconductor physics at about 33%. The coal plant is farther from its limit. Comparing efficiencies across different physical processes without knowing the limits is comparing apples to engines.

Efficiency is the ratio of useful energy output to total energy input: η = E_out / E_in. It is always less than 100% because every real conversion loses some energy to heat, friction, or other unusable forms. For heat engines (coal, gas, nuclear plants), the second law imposes a hard ceiling: the Carnot limit. For non-thermal conversions (solar PV, wind turbines, batteries), different physical limits apply, and the Carnot limit is irrelevant.

This means "efficiency" is not a single standard. A 60% efficient gas turbine and a 22% efficient solar panel are both performing well relative to their respective physical limits.

Worked Example

A combined-cycle gas plant burns fuel with 800 MW of thermal energy and produces 480 MW of electricity.

• Calculate efficiency. η = 480/800 = 60%.
• The Carnot limit for this plant (1,400°C source, 30°C sink) is 82%.

How close is this plant to the theoretical maximum?

60/82 = 73% of the Carnot limit. Real-world losses (friction, heat leakage, turbine blade imperfections) account for the gap. No engineering can close it entirely, but incremental improvements in materials and design push closer.

Efficiency is only meaningful when you know what physical limit governs the conversion.

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