What is the heat output of a computer?
It's hard to get your heed around, but all the electricity that goes into your computer is turned to heat. If you know how much energy your computer draws in watts, you can calculate the heat output in BTUs. BTUs = Watts x 3.412
If you know how much energy your computer consumes in Amps, you can convert amps to watts with the following: Watts = Amps x Volts. This should be listed on the power supply.
Don't be confused by the DC rating of power supplies. The above calculations refer to the amount of AC your computer takes in from the wall socket, not how much the power supply delivers inside the computer as DC.
Because computers have sophisticated power saving features, the idle current, the average current, and the rated current will vary significantly. Idle current can easily be 1/2 the rated current.