Which of the following calculates power dissipated by a series linear voltage regulator?
Why In a series linear regulator the pass transistor sits between input and output carrying essentially the full load current, and it drops whatever voltage the load does not use. Power in any element is voltage across it times current through it, so P = (Vin - Vout) x Iout. That is why a 12 V input feeding a 5 V, 1 A load burns 7 W of heat in the regulator and needs a heat sink.
Watch out Output voltage times output current is the useful power delivered to the load, not the waste heat in the regulator, and input voltage times input current is the total power drawn from the source (load power plus dissipation).
Only the voltage the regulator throws away gets turned into heat: P = (Vin - Vout) x I.