Small-scale horizontal axis wind-turbines (SHAWTs) are acquiring relevance within the regulatory policies of the wind sector aiming at net-zero-emissions, while reducing visual and environmental impact by means of distributed grids. SHAWTs operate transitionally, at Reynolds numbers that fall between 10^5 < Re < 5·10^5. Furthermore, environmental turbulence and roughness affect the energetic outcome of the turbines. In this study, the combined effect of turbulence and roughness is analysed via wind tunnel experiments upon a transitionally-operating NACA0021 airfoil. The combined effects cause a negative synergy, inducing higher drops in lift and efficiency values than when considering the perturbing agents individually. Besides, such losses are Reynolds-dependent, with higher numbers increasing the difference between clean and real configurations, reaching efficiency decrements above 60% in the worst-case scenario. Thus, these experimental measurements are employed for obtaining the power curves and estimating the annual energy production (AEP) of a 7.8 kW-rated SHAWT design by means of a BEM code. The simulations show a worst-case scenario in which the AEP reduces above 70% when compared to the baseline configuration, with such a loss getting attenuated when a pitch-regulated control is assumed. These results highlight the relevance of performing tests that consider the joint effect of turbulence and roughness.