Heat pumps (HP) represent an attractive option as a heat source, since they utilise "free" ambient heat. However, as they need to be driven by electric energy, their ecological efficiency depends not only on their performance, but also on the energy mix used for electricity generation. Effectiveness of HP can be improved by applying renewable energy sources (RES) providing electricity to drive the system. In order to properly assess such a system, it should be evaluated within a global balance boundary reaching the level of primary resources. The authors propose the use of thermo-ecological cost (TEC) indices for exergo-ecological assessment. TEC is defined as cumulative consumption of non-renewable exergy connected with the fabrication of a particular product. TEC also includes additional consumption of non-renewable natural resources which results from the necessity of compensating environmental losses caused by rejection of harmful substances to the environment. Within the paper, a HP system has been analysed from the ecological point of view. The system consists of a heat pump driven by electricity, and photovoltaic panels or wind turbines as a basic source of electricity. The balance of electricity demand for HP is complemented by electricity from grid. To evaluate this system, real environmental data on solar radiation, wind and ambient temperature deciding on heat demand have been taken into account. The authors present the ecological benefits resulting from supporting heat pumps by electricity generated in RES. It has been demonstrated by comparing results obtained within local and global balance boundaries, that the second approach should be consequently applied when systems based on a mix of renewable and non-renewable energy are analysed, since the first approach may lead to misleading conclusions. (C) 2018 Elsevier Ltd. All rights reserved.