Ray-tracing tools are commonly used to optically characterize solar concentrators, but the International Standards used to certify collectors for heating do not allow the use of these tools to analyse the optical behaviour of solar thermal systems. Solar concentrators for the medium temperature range often are of large dimensions and cannot be easily reoriented to the sun without an expensive rotating test platform suggested by the International Standards; therefore, some deficiencies can be detected if the standards procedures are applied to these types of concentrating collectors. In this paper, the use of ray-tracing tools combined with thermal experimental data is proposed to determine the energy balance coefficients by a Weighted Least Square adjustment (WLS). The main advantages of this methodology are that the measurement of the thermal efficiency at normal incidence and solar concentrator reorientation are not required, the optical behaviour of the system can be determined for any position of the sun, and it can be used for in situ measurements for large-scale solar thermal collectors. (C) 2015 Elsevier Ltd. All rights reserved.