Have you any experience with these systems sfag?
I have carefully monitored the cost of heating my cylinder water and it is so cheap there is no point in providing a solution. Thats the bottom line.
Life experience has thought me that any system which still needs another system as backup is a waste. You will never be able to quantify its contribution. In theory you can but not in practice. In theory air ionisers worked - in practice they were snake oil.
You have got to think about it.
These solar things cost serious money.
You only need sun in the winter. We dont get much sun in the winter.
Most days its cloudy. Some days it will be sunny for a few hours. Today there is no sun.
I have large facing windows on the south side of my house.
For a few hours of winter sun they heat up the house in the day. That heat will last for a hour after the sun disappears.
Thats good for weekends when i'm there during the day and I suppose it means my 24/7 ufh doesent have to work during the day when I'm not there.
But there is no science to windows - they have been around for centuries. Their solar gain is a secondry benefit as - as far as I'm concerned - their reason for being is architectural.
If you want solar benefit get the greens to insist on all new builds having south facing windows.