An 800 watt kit is sold on one number. What lands on your bill depends on which wall it hangs on, how it hangs, and how much your flat is drawing while the sun is up. Turn your balcony to its real direction and watch the year change.
Yields modelled for Berlin Mitte with PVGIS (European Commission JRC), clean sky, nothing shading the panel. A courtyard balcony with a building opposite will sit well under these numbers. The flat is modelled as a steady background draw, and everything above it leaves at the standard unpaid feed-in. Above 960 Wp of module the current product standard wants a proper feed-in socket instead of a Schuko plug, so the three larger sizes carry an electrician with them.