This paper aims to present the fundamental problems arising in case of photovoltaic (PV) systems modelling. The goal of these systems is to extract the maximum energy. There are determined the maximum power point coordinates - maximum voltage (U-OPTIM) and current (I-OPTIM), that are basic sizes for determining the maximum load resistance. The proposed control method is characterised by increased efficiency compared to the classic one based on generated power measuring. (C) 2014 Elsevier Ltd. All rights reserved.