How does Wattage relate to heat in a cabinet?

Heat is measured in BTUs and power is measured in Watts. Almost all electrical energy used in computing is converted to heat. A computer power supply can be as low as 80 percent efficient. This means that for every 100 Watts it draws, 20 Watts may be converted directly into heat without ever being used by the computer. As the computer processes information, the rest of the power is dissipated throughout the system as heat. Since all power can be counted as heat, adding the Watt ratings of all equipment in a cabinet will give a relatively 1:1 relationship to heat generated. Example: 40 servers x 300 Watts each = 12,000 Watts (12kW) heat. Also, Watts can be converted to BTUs by multiplying Watts by 3.412.