A separate study by Litjens et al. (2018), further argued that the emission reduction of GSHPs was mainly influenced by a high carbon intensity of the generation mix used to produce the electricity. As such, the researchers advocated for the use of photovoltaic systems (PVs) to facilitate the further reduction of emissions of GHGs in GSHPs. The results from the study showed that combining GSHPs with PVs led to a reduction of GHGs by at least 80% over a period of 30 years which was equivalent to 73 tCO2-eq. However, dwellings that had only acquired GSHPs were estimated to record an increase in NPVs of around €275 per tCO2-eq avoided while dwellings with GSHPs and PVs recorded a lower NPV of about €275 per tCO2-eq avoided. The conclusion was that the ground source heat pumps were not economically feasible for most households in the Netherlands.