For inverters, rectifiers, etc, you should be able to find the maximum input amps required for rated output. If you calculate the input VA then subtract the rated output in kVA, the rest can be assumed to be heat.
For most electronic equipment such as radios, computers, etc, you can assume all kVA at the input will end up as generated heat. That's conservative, obviously.
If you can actually measure the input current when the equipment is running, you'll get a good feel for the probable losses. But unless the equipment is doing some type of energy conversion such as rectification, nearly all the watts into electronic gear turn into heat eventually. There isn't much other place for the energy to go.
Most radio transmitters have efficiency of 50% if that, but duty cycle can be quite low.
