Plug-In Hybrid Electric Vehicles (PHEVs) are a promising solution to reduce fuel consumption and emissions, due to their capability of storing energy in the battery through direct connection to the grid. In order to achieve the highest benefits from this technology, a suitable energy management strategy that optimizes the vehicle energy efficiency must be defined. The present work proposes a supervisory controller for PHEVs, which explicitly accounts for the on-board electricity consumption during vehicle operations. The approach is based on the formulation of an optimal control problem that is solved by the Pontryagin's minimum principle to produce a solution that can be implemented on-line. Simulation results are presented to illustrate the developed energy management strategy.