Look up the Carnot cycle and its efficiency. This is theorized to be the best possible efficiency for a heat engine. All heat engines work by rejecting a large

Look up the Carnot cycle and its efficiency. This is theorized to be the best possible efficiency for a heat engine. All heat engines work by rejecting a large fraction of their input heat to an output heat sink. I've heard efficiencies in the 30-50% range for big coal and nuclear plants, much lower for smaller plants.