An apparatus and a method for determining a power value of a target in the form of an AC circuit (130; 230; 330) having an AC power source (132; 232; 332). The method involves operating (72) a controllable DC power source (12) to provide DC power to a DC circuit (10; 110; 210; 310) and measuring (73) at least one thermal parameter related to power dissipation of the DC circuit (10; 110; 210; 310) and of the target AC circuit (30; 130; 230; 330), wherein at least one heat sink (160a, 160b; 260; 360) is thermally coupled between the DC circuit (10; 110; 210; 310) and the target AC circuit (30; 130; 230; 330). The method further involves controlling (74) the DC power source (12) based on the measured at least one thermal parameter to reduce a difference in power dissipation between the DC circuit (10; 110; 210; 310) and the target AC circuit (30; 130; 230; 330). The method then involves, when thermal equilibrium is reached (75), determining the power value (49) of the target AC circuit (30; 130; 230; 330) by retrieving (76) at least one real-time measurement of at least one electric parameter of the DC circuit (10; 110; 210; 310), calculating (77) a DC power value of the DC circuit (10; 110; 210; 310) based on the retrieved at least one real-time measurement of the at least one electric parameter, and calculating (78) the power value (49) of the target AC circuit (30; 130; 230; 330) using the calculated DC power value.