The usage of conventional energy applications generates disproportionate emissions of greenhouse gases and the consumption of part of the energy resources available in the world. It has become an important problem which has serious effects on the climatic change. Therefore, it is crucial to reduce these emissions as much as possible. To be able to achieve this, renewable energy technologies must be used instead of conventional energy applications. Solar Photovoltaic (PV) technologies do not release greenhouse gas emissions directly and can save more than 30 million tonnes of carbon per exajoule of electricity generated relative to a natural gas turbine running at 45% efficiency. Shadowing is one of the most important aspects that affects the performance of PV systems. Consequently, many investigations through this topic are being done in order to develop new technologies which mitigate the impact of shadowing during PV production. In order to minimise the impact of shadowing it is desired to be able to predict the performance of a system with PV-modules during shadowing. In this thesis a simulation program for calculating the IV-curve for series connected PV-modules during partial shadowing has been developed and experimentally validated. PV systems modelling and simulation in LTspice environment has been presented and validated by means of a comparative analysis with the experimental results obtained in a set of tests performed in the laboratory of Gävle University. Experimental measurements were carried out in two groups. The first group corresponds with the experiments done in the string of six modules with bypass diodes while the measurements of the second group have been performed on a single PV module at HIG University. The simulation results of both groups demonstrated a remarkable agreement with the experimental data, which means that the model designed at LTspice supposes a very useful tool that can be used to study the performance of PV systems. This tool contributes to the investigations in this topic and it aims to benefit future installations providing a better knowledge of the shading problem. The master’s thesis shows an in-depth description of the required method to design a PV cell, a PV module and a PV array using LTspice IV and the input parameters as well as the needed tests to adjust the models. Moreover, it has been carried out a pedagogical study describing the effect that different shadow configurations have in the performance of solar cells. This study facilitates the understanding of the performance of PV modules under different shadowing effects. Lastly, it has also been discussed the benefits of installing some newer technologies, like DC-DC optimizers or module inverters, to mitigate the shadowing effects. The main conclusion about this topic has been that although most of the times the output power will be increased with the use of optimizers sometimes the optimizer does not present any benefits.