It's 800 watts or about 570 KWh per year in berlin, assuming balcony points directly south. At 0.2 euro, that's about 114 euros per year.

Percent-wise I'd guess it's less than 10% of yearly total?

Overall it's OK payback, but mostly penny pinching in grand scheme of things.

https://globalsolaratlas.info/map?c=48.886392,9.470215,6&s=5...