There are various tools to estimate the output in kWh / year of a photovoltaic system. The best is undoubtedly the simulator of the European Community called PV-GIS which allows very accurate simulations at any latitude and longitude in Europe and Africa.
The link access to PV-GIS Europe is as follows: http://re.jrc.ec.europa.eu/pvgis/apps3/pvest.php.
On the left you can select a location by searching either by name or geographic coordinates.
On the right you can enter the technical data of the photovoltaic system of which you want to determine the energy production in kWh unit. You can leave the simulator PV-GIS to determine what are the optimal orientation and tilt of the location indicated.
The simulation is performed based on solar irradiation data collected during the last 9 years. It is therefore a simulation fairly reliable.
Anyway one must take those data carefully. Some recommendations on the use of those data are necessary since the production of a photovoltaic system depends on many factors which are often difficult to be determined. As for example the average temperature of the solar cells etc. ... The shadowing. PV-GIS does not take into account possible sources of shade etc ....
The simulation result is very comprehensive and provides information on both irradiation and energy produced monthly / annual in kWh.