Most PCs typically contain useful processor power management settings in a CPU configuration menu, they should be enabled in the BIOS!
Activation as an example reduces the power consumption of your CPU by reducing the voltage and clock frequency when there is little to do. Not only AMD's "Cool'n'Quiet" provides more accurate CPU speed control. Even modern graphics cards, memory, network cards, WiFi, etc. do not ignore the setting if they support energy-saving mode.
The exact savings you can make by changing these settings depend on your hardware and usage behavior. However, you can see that the power drops by 40 to 60 watts when the PC idles. If your desktop system is on all the time, 95 percent idle, which is not uncommon, you could save a few dollars a year.