2 Rules of Thumb that I was taught years ago as a designer:
1)Every thermal trip element in a circuit breaker puts off roughly 3 watts of heat per pole. Supposedly solid state trip elements don't do that any longer, but that would typically be only on big expensive 3 phase breakers anyway. I still use that rule even if the main is solid state.
2)Breaker contacts are considered to be 99.97% efficient, so look at the total connected kW load and multiply by .0003 for losses in the panel.
Combine the 2 for a total heat loss.
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Hoffman Engineering (the ones that make the popular NEMA style enclosures)has some free software for heat calculations. It is pretty easy to use, but it is based more for large control enclosures, panels w/computers, etc. Download it free from their website.