Yeah my second sentence was a non sequitur, but my point was: you can only do a little with solar/water heating. Yes heating a pool is one of those things, but that's a very niche application. This thread is about heating a house, or at least about being comfortable in one, and solar/water is just not a viable technology for that in the vast majority of cases.
Oh... you said "PV" in both sentences, which is specifically converting light to electricity; that's what threw me.
For heating a house with solar (not PV) -- it would depend on insulation level, I'd imagine. I do know people who use it for "most of the time" water heating, with good results.
