Every element of a circuit can either deliver energy (the source) or absorb it by dissipating it as heat (the resistor). Conservation of energy imposes a precise constraint: in steady state, the total power fed in by the sources must equal the total power dissipated by the resistors. This balance is not a new law — it is derived from Kirchhoff — but it is a valuable checking tool, often quicker when the problem data are powers rather than resistances.