Question: I refer to a 2N3055 power transistor, the spec. sheet gives the following: 

Max. current 15A, Max. Voltage 50V, Power rating is 115 Watts. 

* What is the power rating calculated on?
* If it is E\*I then the power would be 750 Watts, this is of course incorrect, so how is it calculated please.?

Answer: 
The power rating is based on the thermal design of the TO-3 transistor package, how easy or hard it is to extract heat from the transistor die inside. The MJE3055 is basically the same die in a smaller TO-220 package, and is rated for only 75 W. The thermal path from the die to the mounting surface is better than the old TO-3 package, but that mounting surface is significantly smaller.
Total Power Dissipation @ Tc = 25°C
Derate PDmax=115W Above 25°C by 0.657 W/°C for case temp, Tc so **Rjc= 1/0.657= 1.52'C/W**

This implies the max junction temp, **Tj = 175'C+25'C=200'C** but this accelerates failure rate significantly, so a **good design has Tj<< 100'C**.

That assumes an infinite heatsink which is not possible.

So to compute Tc , you need Tja heatsink rating like 0.1 'C/W for a super CPU cooler with Fan or 1'C/W for a large passive heatsink then add internal ambient rise above room temp (or outside temp)

Then Temp rise of junction is Rja\*W= Rjc+Rca(total) \* Watts = Tj rise