If a variable load is to be matched to a given power source in order to achieve the maximum possible power dissipation in the load, the resistance of the load must be made equal to the internal resistance of the source. The available power then obtainable is V_o²/4R_i, where V_o is the open-circuit electromotive force of the source and R_i the internal resistance.

The converse problem, that of matching a power source to a given load in order to achieve maximum power dissipated in the load, is not solved by this theorem. In this case the power source with the lowest internal resistance gives the maximum power.

This theorem can be modified to apply to alternating-current linear networks. The impedances Z_S and Z_L of the voltage generator and load, respectively, contain an imaginary term:

For maximum power dissipation in the load it is necessary to satisfy the conditions given by

When this applies the circuit is said to be conjugate matched.