In Nordic countries, interest is growing in the use of photovoltaic/thermal (PVT) collectors as sources for borehole regeneration in ground source heat pump systems. A PVT collector is the combination of a photovoltaic (PV) panel and a solar thermal collector. The benefit of this hybrid system is the solar thermal collector’s cooling of the PV panel, allowing for the simultaneous generation of electricity and heat. It is possible to increase the heat gains of the system if the temperature of the working fluid is under that of the ambient temperature. This allows for heat gains even at times of no irradiance, improving the thermal performance of the system. In order to optimise heat exchange between the working fluid and the ambient, fins on the back of the PVT absorber are used to increase the heat exchange surface area. This study compares the thermal performance of two prototype box-channel PVT collectors, assessing the impact of fins in low temperature operating conditions, PVT inlet temperature being lower than ambient. The two PVT collectors are tested simultaneously at an outdoor testing facility at KTH Royal Institute of Technology in Stockholm, Sweden. The outdoor testing environment allows for the analysis of a variety of different weather conditions, such as solar irradiance as well as wind speed and wind direction. The impact of different working fluid flow rates and roof installations on the thermal performance are also evaluated. Quantifying the impacts of these different condition will enable the evaluation of if the increased performance of the fins justifies the additional costs they represent. To obtain thermal performance coefficients, a simplified version of the equation for modelled thermal power of a solar thermal collector presented ISO 9806:2017 standard is used. These coefficients are used as key performance indicators to compare the thermal performance of the two PVTs under the different conditions studied. The results show that increasing irradiance and wind speeds improve the thermal performance of the PVTs. However, it was observed that the same increase in either irradiance or wind speed does not result in the same improvement in specific thermal power under all operating conditions. An optimum flow rate of 0.021 kg/s/m2 was identified. Additionally, it was found that fins do improve the thermal performance of the PVT collectors. Improvements in specific thermal power, when compared to the PVT with no fins, as larger as 16% were observed, at a flow rate of 0.028 kg/s/m2 . When the airflow around both PVTs was restricted, through the addition of roof installations, this improvement in specific thermal power dropped to 6 or 7% depending on the roof installation.