Current codes of practice for PV systems lack detailed guidance regarding circuit mismatch, over or reverse current protection and unbalanced operational conditions in large PV systems. Experimental work in this field is expensive and limited by hardware and environmental resources. The available commercial simulation tools do not rigorously model the complex behaviour of PV systems operating under non-uniform conditions. In this paper a detailed cell-by-cell model of large scale PV systems is developed. The parameter set used for simulations is based on real PV modules power tolerance data and the variance in its principal parameters, thus representing a realistic power frequency distribution. The model is used to estimate and analyse losses due to circuit mismatch, analyse the causes of reverse current in the system's strings and its consequences in the system performance and to estimate energy losses due to string's fuses failures. (c) 2015 The Authors. Published by Elsevier Ltd.