There are a couple of things at play here. First of all, and most importantly, the quality of the panels you are getting for your 20k€, and thus the total amoun

There are a couple of things at play here. First of all, and most importantly, the quality of the panels you are getting for your 20k€, and thus the total amount of power they'll produce over their lifetime has gone up dramatically. Secondly while the cost of the panels has come down slightly, the cost of getting people to install them hasn't, and that has on the whole increased. If you where to install the solar panels as part of of either building the house or replacing/repairing an old roof that needed fixing anyway then it will be significantly cheaper.