An apparatus 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 apparatus comprising: a DC circuit (10; 110; 210; 310) comprising a controllable DC power source; electric measuring means for measuring at least one electric parameter of the DC circuit (10; 110; 210; 310); at least one heat sink (160a, 160b; 260; 360) thermally coupled between the DC circuit (10; 110; 210; 310) and the target AC circuit (30; 130; 230; 330); thermo-sensory means for measuring 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); and a balancing unit having: an input operatively connected to the electric measuring means; at least one input operatively connected to the thermo-sensory means; and an output coupled to the controllable DC power source, wherein the balancing unit is configured to: control the controllable DC power source based on the at least one thermal parameter measured by the thermo-sensory means to reduce a difference in power dissipation between the DC circuit (10; 110; 210; 310) and the target AC circuit (30; 130; 230; 330); and when thermal equilibrium is reached, determine the power value of the target AC circuit (30; 130; 230; 330)) by retrieving at least one real-time measurement of the at least one electric parameter of the DC circuit (10; 110; 210; 310) from the electric measuring means, calculating 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 the power value of the target AC circuit (30; 130; 230; 330) using the calculated DC power value. a DC circuit (10; 110; 210; 310) comprising a controllable DC power source; electric measuring means for measuring at least one electric parameter of the DC circuit (10; 110; 210; 310); at least one heat sink (160a, 160b; 260; 360) thermally coupled between the DC circuit (10; 110; 210; 310) and the target AC circuit (30; 130; 230; 330); thermo-sensory means for measuring 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); and a balancing unit having: an input operatively connected to the electric measuring means; at least one input operatively connected to the thermo-sensory means; and an output coupled to the controllable DC power source, an input operatively connected to the electric measuring means; at least one input operatively connected to the thermo-sensory means; and an output coupled to the controllable DC power source, wherein the balancing unit is configured to: control the controllable DC power source based on the at least one thermal parameter measured by the thermo-sensory means to reduce a difference in power dissipation between the DC circuit (10; 110; 210; 310) and the target AC circuit (30; 130; 230; 330); and when thermal equilibrium is reached, determine the power value of the target AC circuit (30; 130; 230; 330)) by retrieving at least one real-time measurement of the at least one electric parameter of the DC circuit (10; 110; 210; 310) from the electric measuring means, calculating 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 the power value of the target AC circuit (30; 130; 230; 330) using the calculated DC power value. control the controllable DC power source based on the at least one thermal parameter measured by the thermo-sensory means to reduce a difference in power dissipation between the DC circuit (10; 110; 210; 310) and the target AC circuit (30; 130; 230; 330); and when thermal equilibrium is reached, determine the power value of the target AC circuit (30; 130; 230; 330)) by retrieving at least one real-time measurement of the at least one electric parameter of the DC circuit (10; 110; 210; 310) from the electric measuring means, calculating 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 the power value of the target AC circuit (30; 130; 230; 330) using the calculated DC power value.