We asked for an offer for 6 panels. That would cost € 320 per panel, including installation, all cables etc., and a monitoring system:  to see yourself how many energy in Kwh you are “collecting”,  what quantity you use yourself,  how much you sell to your energy company. However, the choice for 6 panels was just an estimate. I asked Peter, our neighbor, if I could have a look at our own roof from his garden, as his backyard goes much more deeper than ours. Then it was clear, that there was place enough for more panels. PLUS: Peter was enthusiastic about our plans and decided to participate in the project.
I ordered and made a down payment to have the roof inspected. If at the time of the inspection it may be clear, that solar panels will not pay back the costs, negotiations are stopped and you get the down payment back.