One of the central variables in bottom-up energy efficiency and saving calculations is the energy consumption baseline In the evaluation of energy efficiency measures developing this baseline is a challenging task which may involve serious problems especially if the energy service of the analysed subject has changed while the energy efficiency measure was being implemented In this paper we present a formalised concept of the process of developing the baseline that is flexible enough to deal with various difficulties such as changes in the levels of the energy services involved We also discuss the most relevant options for deriving the necessary variables (C) 2010 Elsevier Ltd All rights reserved