If you use electrical power, you always have output heat dissipation.
As long as you consider the maximum values listed on the electronic device’s power supply for your calculations then whatever the electrical power is being used for (running disk drives, memory, driving lamps, controlling a laser) it doesn't matter; it's all going to be converted to heat dissipation.
To approximate electrical power to heat dissipation, you can multiply rated input voltage (V) by rated current (I) to get Volt-Amps. Then multiply by a power factor.
In ideal conditions, power factor is considered as the cosine of the between-the-phase angle (also called “phase shift”), of the measured current and voltage.
Most power factors for electronic equipment are specified as a number between 0.66 and 1.0 with the smaller the number being the more efficient.
Since we are looking for the worst case, the maximum heat dissipation, we should always use the power factor of 1 to be safe.