Cheapest battery, but very expensive to heat. It consumes a lot of energy.
I can survive without gas, heating is electric. I have spent some time without gas and I can manage. My computer and internet connection is more important than heated water. I have backup electric heaters (and an infrasauna).
Electric heating? Solar wont help you with that. The only system-failure resistant heating is imo the wood stove with good accumulation.
But thats a different story.
For FV to be economical you need to use energy as muchas possible when you have it (in summer).
This means that electric boiler is no brainer, you have hot water for zero* cost ~half of the year.
*Of course the is the aquisition cost (high) of FV + its maintanenece costs (low).
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).
Can you share how you get to that number (80-100€/MWh)? What is your expected lifetime of FV system?
Of course if you don't have enough space for panels to have enough output, then having FV only for internet/light/electronics/fridge is a good idea anyway.
Roughly: 1kWp makes 1MWh per year where I live. Lifetime 20 years. So divide the price (without batteries, which double the price) by expected yield over lifetime.
