This paper proposes a new method to accurately estimate the power and energy production in urban photovoltaic (PV) systems, which are commonly covered by shades affecting its performance. The solution is based on an efficient algorithm designed to compute, in short time, an accurate model accounting for the shades impact. In such a way, the proposed approach improves classical solutions by significantly reducing the processing time to simulate long periods, e.g. months and years, but without introducing sensible errors. Therefore, this approach is suitable to estimate the production of PV systems for economical analyses such as the return-of-invested time calculation, but also to accurately design PV installations by selecting the right number of photovoltaic modules to supply the required load power.