I am looking at the datasheet of 2SD1047 transistor which says the transistor is 12A 140V 100W. I want to know how Power is calculated?
If I use this transistor at 12V 10A load how it affects the wattage?
> 12A 140V 100W datas mean this:max. 12 A
collector current, max 140v
collector-emitter voltage but only max 100w
dissipation power (I * U).
On your example (transistor at 12V 10A load) if the transistor is well opened (it has enough basic current), then it's working like a shortcut. In the reality a small (saturation) voltage remains on it. In this case we can be calculate with 2V (see the datasheet diagrams). So the dissipation ('wattage') will:
2v * 10A = 20W
Hope ths helps.