An apparatus, system, and method are disclosed for controlling fan speed in a power supply. The apparatus measures input power to the power supply and measures the output power provided by the power supply over an interval. The apparatus determines values for the input power and output power and, using the two, determines how much power has been dissipated in the power supply. Power dissipation values are associated with particular fan speeds, and the apparatus adjusts the speed of the fan in the power supply based on how much power was dissipated during the interval. Increasing levels of power dissipation increases the fan speed, and decreasing levels of power dissipation decrease the fan speed.