This article introduces the normalised efficiency of photovoltaic modules and systems as a performance monitoring metric, and its similarity to the Performance Ratio is shown. Detection of shading using the normalised efficiency is presented. From module temperature measurements and using the normalised efficiency, it is possible to calculate a system temperature coefficient, which then serves for temperature-corrected system analyses, and may serve to identify otherwise hidden temperature-dependent behaviour in the system studied. A simpler method to obtain coefficients for the modelled normalised efficiency is given which requires only irradiance, module temperature and power data as modelling inputs, and validated using three photovoltaic systems.