Energy storages are key elements for the design and operation of nearly-zero-energy buildings. They are necessary to properly manage the intermittency of energy supply and demand and for the efficient use of renewable energy sources. Several storage technologies (electrochemical, thermal, mechanical, etc.) to be applied at building scale are already available on the market or they are in the final stages of research and development. These technologies have heterogeneous features and performances; therefore, it is necessary to develop a procedure to compare different alternatives in order to carry out a techno-economical assessment. This paper summarizes the current status of energy storage systems at building scale and proposes a set of simplified Key Performance Indicators (KPIs), specifically identified to simplify the comparison of energy storage systems in the decision-making/designing phase and the assessment of technical solutions in the operational phase. The defined KPIs are finally applied to 10 case studies analyzed within the International Energy Agency Energy Conservation Through Energy Storage (IEA ECES) Annex 31 "Energy Storage with Energy Efficient Buildings and Districts".