The potential difference between the ends of a resistance in a circuit is V=R·I where I is the current and R is the value of the resistance (Ohm's Law). 

    The power consumed in the resistance is P= V·I  or rather  P=R·I²   and the energy after a time t will be: W=R·I²·t (Joule's Law)

 We call the energy a generator produces for each unit of charge the electromotive force. It is measured in volts.

We call the energy an engine consumes per unit of charge the counterelectromotive force. It is also measured in volts.

Generators and engines behave as though they had a certain internal resistance in which heat is dissipated.

Ohm's Law generalized to a circuit with generators and motors is: E - E'= R·I+r·I+r'·I where E is the electromotive force of the generator, and E' is the counterelectromotive force of the engine. R is the resistance of the circuit, while r and r' are the internal resistances of the generator and the engine respectively.

The power supplied by the generator is Pg=E·I  and that used in the engine is: Pm=E'·I. The difference between the two powers is lost in the form of heat in the different resistances.