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Solar cells is one of the cleanest and most environmentally friendly ways to produce electricity. Västerås city has invested in a number of solar systems in public buildings as a step to solve the energy issues of the future and contribute to a sustainable environment. The purpose of this project is to compare the real system yield from Västerås city´s solar plants with simulations. Produces the solar plants as expected or not, and if not, what might be the cause. Data were collected about the solar cell installations, by Mälarenergi Elnät and Västerås city. The real system yield was calculated and then the solar plants were simulated in PVGIS (Photovoltaic Geographical Information System) to obtain the theoretical yield. This project shows that most of Västerås city´s solar plants have a yield that is as expected according to the simulations or higher. However, there are some solar plants with a low or very low yield compared to the simulations. The yield varies considerably during the year. For those plants where the yield has been studied monthly, the real yield is higher in the second half of the year compared with the first half. The self-consumption varies greatly between the different solar plants, but generally it is high. Some solar plants have a very high self-consumption of 100 % and some have a very low of 30-40 %. Some solar plants have a higher yield than expected and it may depends to the fact that the installed power is a few percent higher than what the manufacturer states. The simulations might be unsure, because losses, solar radiation and weather may vary. The solar plants that have a low yield compared to the simulations may have a broken or disconnected component, shading and dirt may also affect. To have as high self-consumption as possible is an economically advantage, as long as the plant is not under-dimensioned because the goal is to produce electricity. There is no clear pattern showing which of the four PVGIS simulations is best matched to reality.