For reproducible and interpretable photocatalytic experiments, it is crucial to determine the light power that is incident on the photoreactor. To this end, a power measurement is performed before each experiment, where a power sensor is placed in the position of the photoreactor. For these measurements we are using a thermopile power sensor, which is characterized by a relatively flat response across the UV/visible wavelength range. This makes it especially suitable to measure output power from broadband light sources, such as white LEDs.