The present study focusses on the thermal performance representation and testing of air solar collectors with an emphasis on flowrate and leakage effects. The thermal efficiency was evaluated using two methods. In the first method, referred to as the oulet mass flowrate method, the efficiency is based on the product of the outlet mass flowrate, the air specific heat and the temperature rise across the collector. The second method, called the enthalpy balance method, is based on the first law of thermodynamics and takes into account all of the energy fluxes into or out of the collector. Both methods give the user all the information he needs as long as the inlet and outlet mass flowrates are reported. But it has to be recognized that each method of representation gives anomalous results depending on the type of leakage. For example, it is possible, in the outward leakage case, for the enthalpy balance method to give a positive efficiency even though there is no useful energy delivered. In the inward leakage case, the outlet mass flowrate method might give a negative efficiency even though the useful output energy is greater than the input energy. A "delivered" efficiency was defined to aid in applying results to real engineering systems and to better represent the energy truly delivered to the load. Experimentally, to take into account the effects of flowrate and leakage rate on the thermal performance of air solar collectors, it is proposed to test air solar collectors at two flowrates and three inlet gage pressures for each of these flowrates. The test facility should include inlet and outlet mass flowrate measurements and a dual-blower system to control the inlet pressure.