Heat pumps are considered one of the most efficient heating and cooling systems and, according to theDirective 28/2009/EC, the aerothermal, geothermal or hydrothermal energy captured by these appliancesis considered energy from renewable sources. For these reasons, they will play a key role in reducinggreenhouse gas emissions, contributing to reach the target of nearly zero energy buildings.However, there is still a great deal of uncertainty on the actual performances of heat pumps in reallife: indeed, the same unit can have very different consumptions depending on local climate, settings andbuilding characteristics. The aim of this article is to investigate these effects, focusing on the applicationin residential buildings where a single device provides both space heating and cooling.A model that simulates the hourly efficiency of air-to-water heat pumps is developed based on ther-modynamic considerations and on data coming from a field trial monitoring campaign. The model isused to predict the behaviour of air source heat pumps installed in a set of residential buildings locatedin different Italian cities, in both heating and cooling mode. The obtained results show that the climateplays the leading role on annual performance. The ratio between heating and cooling peak load is equallyimportant because in buildings where heating load is very different from cooling load, or vice versa, theunit has recourse to excessive cycling in the less severe season, causing a seasonal efficiency reduction upto 25%. Moreover, the benefits coming from a weather compensation strategy are investigated, showingan annual performance improvement up to 19%.