This chapter analyzes energy-saving methods in the thermal systems of NPPs, comparing the differences between the energy balance method and the exergy analysis method. The former emphasizes heat loss (e.g., condenser dissipation), while the latter focuses on exergy losses caused by irreversibility during energy conversion. Improving thermal economy hinges on reducing exergy losses in reactors, heat exchangers, and turbines through measures such as increasing nuclear fuel temperature, minimizing heat transfer temperature differences, enhancing heat exchange technologies, and optimizing equipment design. Rational energy utilization principles (e.g., energy-grade matching and cascade utilization) are proposed to mitigate energy degradation. Current PWRs face material limitations, whereas sodium-cooled fast reactors and HTGRs demonstrate higher efficiency potential due to elevated coolant temperatures.