éco21 is an energy efficiency program addressing the electricity sector in Geneva-Switzerland (population ~460000 and yearly electricity consumption ~2850 GWh/year, CERN excepted). The program was launched in 2007 and was redesigned in the middle of 2009. At the very beginning of the new program, the University of Geneva was requested to develop the evaluation methodology for the program and do the corresponding monitoring and evaluation. After a brief presentation of the program éco21 and the framework of its monitoring and evaluation, we address the evaluation methodologies pertaining to two of the subprograms (Low Income Household Energy Efficiency and Building Communal Areas) and the results of the very first evaluation experiences. For these two subprograms, different options of enhanced engineering estimations – that are to be used with a sample of projects – will enable to refine simple engineering estimations. The aim is to drop the enhanced engineering methods later in favor of the (improved) simple engineering estimations that are more time and cost effective. Billing analysis will be used systematically to double check the estimates and to evaluate the lifetime of the actions.