CC BY 4.0Huang, MuMuHuangWapler, JeannetteJeannetteWaplerHelmling, SebastianSebastianHelmlingIturralde, JonJonIturraldeCivalleri, LorenzoLorenzoCivalleriHengel, FranzFranzHengelHütter, JakobJakobHütterGünther, DannyDannyGünther2025-06-272025-06-272025https://doi.org/10.24406/publica-4824https://publica.fraunhofer.de/handle/publica/48898410.1016/j.energy.2025.13722410.24406/publica-4824Photovoltaic-assisted two-stage heat pump systems—integrating both centralized and decentralized heat pumps—represent a promising solution for decarbonizing existing multi-family buildings. This study aims to conduct the first empirical investigation into the energy performance of these systems based on three case studies from Europe. By establishing comparable energy performance metrics, the necessary field data was collected and preprocessed for an annual assessment. The overall seasonal performance factors of the heat pump systems are similar, averaging around 2.0 across all three case studies. Between 63.5 % and 76.3 % of the primary energy used for heating and cooling comes from renewable energy sources. Compared to previous heating supply systems, greenhouse gas emissions from the heat pump systems are reduced by 65.4 %–84.3 %, fulfilling the European Union’s target of a 60 % reduction by 2030. Between 60.9 % and 86.4 % of the emission savings are attributed to the installation of the two-stage heat pump systems, while 13.6 %–39.1 % of the savings result from photovoltaic electricity self-consumption. The findings demonstrate the technical feasibility of retrofitting photovoltaic-assisted two-stage heat pump systems in existing multi-family buildings across various climate and energy demand conditions, highlighting their potential to significantly contribute to sustainable building practices.enDecentralized heat pumpsExisting multi-family buildingsHAPPENINGLow-temperature distribution loopMonitoringPhotovoltaicjournal article