What is the optimum tilt angle of photovoltaic plants in mid-latitudes? This question is of practical importance for the mounting of photovoltaic systems. The present work states a nearly irrelevant difference of the yearly performance of solar cells at various tilt angles. The measuring system included eight multicrystalline silicium solar cells and was mounted on the roof of the Institute of Meteorology and Climatology (IMuK) in Hannover, Germany, for a 1-year period that started from November 2008 until October 2009. Each solar collector was mounted at a different tilt angle between 0 degrees and 70 degrees in steps of 10 degrees, in a southward orientation. The measurements covered the short circuit current (I(sc)), the open circuit voltage (U(oc)) and the cell temperature (7) of each cell. From this the maximum power (P(mp)) was calculated and analysed. The data has been assessed for monthly sums. Maximum values of P(mp) were found to appear in a wide angular range, about 50 degrees-70 degrees in the winter months and 0 degrees-30 degrees in the summer months. The yearly optimum tilt angle was found to be nearly the same as for summer months. The largest difference of the plant yield was less than 6% for tilt angles between 0 degrees and 70 degrees. This holds for both yearly sum and for the summer months. Theoretical calculations performed with INSEL software, however, showed larger deviations than the experimental findings. This is probably due to temperature effects, which tend to level off differences at different incident angles. Further investigations are necessary to test whether the tilt angle is generally irrelevant or whether other sites or years will show different results. (C) 2010 Elsevier Ltd. All rights reserved.