For transformers, the heat production is readily obtained from manufacturers data. If for example a 500KVA transformer has an efficiency of 98%, then 2% or 10KVA must be lost as heat.
In equipment with relays, contactors, pilot lamps etc, then the power used by these devices all ends up as heat, and an estimate of the heat produced could be obtained from the input rating of the control circuit transformer(s)
Losses also occur in fuses, and of course these losses end up as heat. Here in the UK standards exist as to the maximum watts lost in various fuse types (primarily to protect against overheating of the fuse carrier) and I would presume that similar standards exist in the USA.
Losses also occur in cables, busbar, circuit breakers etc, these are not so readily determined, and may have to be measured.
For example if the voltage into a switchboard is 480 volts, and the output is 479.5 at a load current of 1,000 amps, then the losses are clearly 500 watts, per phase.
Remember to add the heat input from room lighting, and from any other equipment that is installed.
No great accuracy can be expected, and a generous margin of ignorance should be allowed.
