Performing measurements and compiling a power matrix (IEC 61853-1) is a useful tool for illustrating the energy production of a PV module at different levels of irradiance and temperature. At the European Solar Test Installation (ESTI) a steady-state solar simulator can be used to determine these matrices. The steady-state solar simulator irradiates the PV module from the front side, which heats the module to the desired temperature. However, this causes a temperature difference between the front surface of the module and the rear, where the temperature sensor is attached. This temperature difference, between the measured module temperature (Tm) and the temperature of the cell junction, gives rise to errors. In this paper, a correction procedure according to the determination of the equivalent cell temperature (ECT) of photovoltaic devices by the open-circuit voltage method in IEC 60904-5 is proposed and evaluated. In this study the difference between the Tm and the ECT was determined to about 1 °C to 3 °C, resulting in a deviation of the Pmax value in the power matrix up to about 1.5%, for a monocrystalline PV module.