The cost of electricity from fv if you use it all is 80-100€/MWh over the lifetime of the system in central Europe.

The form of up front payment is irrelevant, it's never free, unless you are not able to use it all. I can use it all and don't care much about heated water, I take cold showers and don't need heating in summer. So any outage of hot water does not concern me. Space on the roof is also limited and completely used in my case (for the part that is usable for solar, half of it is in shade most of the day).