Evaluating operating costs is a key step in the successful development of an aerospace project. In particular, maintenance costs represent a very important cost voice for aircraft and many tools for estimating them since the preliminary design phase are publicly available. Since new aviation tendencies and technological improvement are pushing towards the developing of More Electric Aircraft (MEA), new cost estimating relationships need be built up in order to replace the conventional ones, that can no longer be exploited in these cases. The objective of the present work is to provide an estimation relationship which is able to evaluate the amount of scheduled Maintenance Man Hours (MMH) that are saved or added, when going from a conventional On-Board System (OBS) architecture to a more or all electric one.