Upgrade the Power Supply
When it comes to wasting power, perhaps the most logical culprit is the power supply. An inefficient power supply can cause your computer to draw more power from the wall than its wattage rating states, because a good portion of that power is lost to heat.
Before 2005, PC power supplies might have been just 60 percent efficient [source: Won]. That means that for a computer that needs 100 watts to operate, the power supply needs to pull 167 watts of power, because 40 percent of that is going to be lost as heat.
Since 2005, though, efficiency is up above 80 percent [source: Won]. So that same computer will only have to draw 125 watts of dirty grid power from an outlet in order to provide the 100 watts of power the PC needs to run. That's a 25 percent decrease in energy use just from updating the power supply to a more efficient model.
Depending on how much power you're looking for, you can spend anywhere from $30 to $100 on a new unit. It's an easier upgrade than the CPU, financially speaking.
And at No. 1 on our list, the single greatest power draw in a computer system.