Maximum power across a transistor
The power dissipated by a transistor in a circuit is determined by the usual power formula: P = E x I - that is, the power is the voltage across the transistor times the current through the transistor.

The maximum recommended power dissipation for a transistor will be found in the data sheet, under "Total Device Dissipation", or words to that effect.  There may also be a graph of power dissipation vs ambient temperature.