In this paper we study the energy budget during magnetic reconnection. To this end, we concentrate on time-varying reconnection. Our considerations are based on the generalization of the Petschek model for a time varying reconnection rate. For the sake of simplicity, we consider a 2D asymmetric current sheet configuration with an incompressible plasma. Under these circumstances, the reconnection problem can be formulated in an analytical way, and a particularly clear insight can be gained. It turns out that as a result of the fast motion of the outflow region, magnetic energy is accumulated external to its leading front, whereas it is rarefied in the wake region.