A Bipolar Junction Transistor (BJT) is a common device used to amplify or switch electronic signals and electrical power. When a BJT conducts current between its collector and emitter, it will dissipate power.
How do you estimate the BJT’s power rating? 
Power Rating is the maximum amount of power that a device can dissipate in a typical operating environment.
BJT Power Rating = Collector Emitter Voltage * Collector Current + Base Emitter Voltage * Base Current
For example, to estimate the power rating of Micro Commercial Co (MCC), BC856BM-TP.
Refering to the datasheet, it estimates BC856BM-TP power rating (at 25°C.)
= Collector Emitter Voltage * Collector Current + Base Emitter Voltage * Base Current
= ~ 65V x 100mA (where Base current is very very small and can be neglected)
= ~ 6.5W
Please note that a BJT’s power rating will change depending on ambient temperature and mounting surfaces.