Due to environmental effects of extensive use of energy, solar collectors are being installed on a large scale in many countries. The accurate determination of its efficiency by a test laboratory strongly affects the system performance prediction, cost-benefit valuations, technology introducing in public policies and certification programs. In this context and considering that indoor efficiency tests using solar simulators are increasingly used by test labs, this paper presents an assessment of uncertainty associated with usual flat plate solar collector efficiency tests performed with solar simulator. For this purpose, tests were performed according indoor method in order to estimate uncertainty associated with efficiency determination. Uncertainty evaluation included the contributions of instrumentation, temporal stability of environmental test conditions and statistical regression used to determine collector's efficiency curve. Additional tests and evaluations were also carried out to investigate solar simulated irradiance and air speed influence on estimated uncertainty and to validate the results from comparison with uncertainties derived from two other laboratories. (C) 2016 Elsevier Ltd. All rights reserved.