I think you are severely overestimating the thermal resistance of floor heating. Your house seems to need only a couple kW at most of heating to maintain equilibrium, across 65m2 floor heating if not buried deep in a slab of concrete can supply that down to a couple of degrees difference (see page 16). There's the edge case of pump failure, but thermal inertia has nasty edge cases as well ... if you arrive at the house when heating failed it will take much longer to warm up and worst comes to worst and you have to switch to a wood fire all that thermal inertia really becomes your enemy.
Not sure you understood the point I was trying to make with the floor thermal storage vs water thermal storage.
The usable range for floor thermal storage is much smaller and the extreme limits that it can be used is 30C max (that is the spec for any type of floor heating) and minimum is 18C for comfort thus the 12C delta range for max usable capacity.
For water storage if you say have a steel storage tank you can go from 20C up to 80C or closer to boiling point.
Say your house has no floor thermal storage or very little like just minimum concrete thickness needed for floor heating and you have all the thermal storage calculated for say worst case 6 days but then you get a year where you get 8 bad days.
Say both storage are 180kWh and house needs 30kWh/day (say this is for -10C out + 20C inside) (just round numbers for this example).
Now if all your storage if the concrete floor and you have 180kWh at 12C delta (18C to 30C) typical ambient may be 2C lower than floor temperature
And water storage 180kW at 60C delta (20C to 80C).
Now both will deal with those 6 days without any solar (is just for the example there is no such thing as no solar output unless at north pole).
While both have 180kWh usable after that is spend the remaining energy before house will freeze inside (0C) is 60kWh about 2 days theoretical for water storage but about 270kWh before concrete floor gets to 0C freezing.
There are many situations here in Canada and maybe other cold places where the heating fails and since the house has no thermal storage at all (wood frame houses) the pipes inside the house for water and or heating will just freeze in a few hours and that will be very costly and inconvenient.
A house with proper thermal mass to work with solar will only lose maybe 2C in 24h if heating fails thus there are quite a few days even a week for you to fix the situation.
There is nothing to fail on my heating as I have electric heating wires embedded in the floor (solid state nothing to fail) and if say the DMPPT450 fails for some reason you just remove that and connect the PV directly to the heating wires a bit less efficient but it will still work indefinitely until the DMPPT450 gets fixed or replaced.
Wires can not fail as current is limited both by the PV panels that are constant current limited and the wire resistance in case of heating wire so there is nothing more reliable than direct PV heating.
Sorry for getting over complicated with the answer.
The short answer is that concrete floor has larger margins and best is to combine floor thermal storage with water thermal storage as I will have as then you can control the temperature much better and be able to keep the house within 1 or 2C delta at all times.
I always pondered on the feasibility of using PV for electric, vs thermal solar panels. PV is simpler, because you just have one big system, that also powers everything else. With thermal you would need some sort of liquid like glycol or something and there's just more moving parts and risk of leaks and environmental issues etc. Also don't know if you'd get ANY heat when it's overcast. With PV you still get some power output.
It's my dream to one day buy a big property so I can do stuff like this and live mostly off grid. I'd do a large ground mount array, like 10-20kw maybe even more. WAY cheaper in the long run, as utility bills and costs of living keep going up. My current property is just too small to do a decent size PV array though. I did setup 400w of solar panels on my shed to experiment and get around 60w or so on a good day. It's facing west as that is all that was really viable due to shadows. But it's cool to know that even when it's overcast I still get SOME output. I have not yet my system during a sunny day. We don't really get much sun here (northern Ontario). In summer we do get more though. With big enough inverter could easily run AC.
I still have work to do on my system though such as some monitoring/automation (like low voltage cut off), heated/insulated battery box, etc. For hydrogen mitigation I'll probably build a small HRV system. Eventually it will power outside lights.
Oh and another thing, in an off grid setting you'd probably want wood heat too, nothing like sitting in front of a wood stove on a -40 day watching the storm outside. So even if the PV is not keeping up every day you could still start a fire in the wood stove. Me personally I'd probably also want a force air system just for circulation. In summer when the sun is out and the days are long there would be enough excess power to run a full size central AC unit. The inverter would be expensive though...
Thermal solar is not only less reliable but also more expensive with only advantage that it takes less space about 2.5x less space for same output. You will get some output from thermal solar even when overcast but if is very cold it may only cover the losses so there may not be high enough temperature to get anything usable.
If I will have designed the house I have now I could have integrated all the necessary PV array in to the house structure (probably an A frame type structure to have the right angles for the panels).
Using Lead Acid battery for storage is just a waste of money. There is a replay I made earlier about cost amortization and Lead Acid is just not suitable for energy storage.
I have no backup heating as is not needed and adding a backup system even for 10% of the time is not cost effective. As long as you size the system for worst case PV array size and thermal storage size it will just work in any conditions and will be the most cost effective solution (Wood is not even close in therms of cost that is why I always mention natural gas as that is the one that gets close).
I do not need air conditioning/cooling in summer as the house is well designed and my low energy use means I do not produce much heat inside the house. Most houses are just badly designed with large windows bad roof thermal insulation and use a lot of electricity that all ends up as heat inside the house and needs to be removed.