This work presents a sensitivity analysis of the overall heat loss coefficient UL and the thermal efficiency η in low and medium temperature encapsulated flat plate solar collectors when controlling the output-input temperature difference ΔT and the angle of inclination β. The UL and η were determined by heat flow calorimetry at indoor conditions, emulating the solar radiation by the Joule effect and a PID control. The angle of inclination β range was 0–90 deg, and the ΔT range was 5.0–25.0 K. The ambient temperature and the mass flowrate were preset for each test. The UL experimental uncertainty was ±0.85 W/m2K for the inclination range of 0–45 deg and ±0.27 W/m2K for the inclination range of 45–90 deg. The results matched previous outcomes with a difference of up to 0.3 W/m2K. The UL exponentially increased with β from horizontal to vertical position and linearly with ΔT. The UL and the efficiency were sensitive to the confined airflow variations. The efficiency increased 140% when β was raised and 40% with ΔT.