This paper examines the optimisation of a PV/T air collector with a fixed length and width (L=5 m, W= 3.6 m) linked to the mechanical air distribution system of a typical residential building. A methodology is developed for the full optimisation of the rate of effective thermal output (Qeff) across a wide range of the air mass flow rate per collector unit areas (m/Ac = 0–0.07 kg/s m2). For the optimisation methodology the air temperature rise (∆T) was not restricted and several values of the weighting ratio of heat to electricity were compared. The results show that optimally designed PV/T systems can deliver essentially the same amount of net energy across a wide range of air mass flow rates suitable for a wide range of end-use applications with varying temperature rise requirements. In comparison when an exergy optimisation method is used, the resulting PV/T air system design delivers a low air flow rate and a very high air temperature rise (∆DT), but delivers low amounts of energy. Therefore the exergy method is not recommended for the design of a PV/T air system where the end-use is space heating. In addition a sensitivity analysis of Qeff and ∆T values was carried out across a wider range of the air mass flow rate per collector unit areas (m/Ac = 0.01–0.2 kg/s m2) and various values of the air channel depth (D = 0:01-0:09 m). It was found that a PV/T air system design with a smaller collector depth offers a good performance at higher ∆T but this design is very sensitive to changes in the air mass flow rateper unit collector area (m/Ac). On the other hand, a PV/T air system with a larger collector depth can deliver a better overall energy output but a lower temperature rise is achieved. However, the system is less sensitive to variations in the air mass flow rate.