I got a VELUX flush-mount system quoted out a couple months back, figuring with my 70% (PA+FED) it would be no brainer. My installer wanted $13k for a two panel system and $15k for a three panel. After
rebate that is $3-4k for me.
I decided against it after looking at my monthly insolation numbers from this website:
http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/
Looks like my solar resource is good from a hours of peak output perspective (>1000 hrs/yr), but is basically hopeless in the winter months, due to clouds. When I looked at the Velux system, the storage is pretty meager b/c the tank always needs to have some hot water for a call, but also needs to store extra when solar input is high. Basically, the backup coil kicks on when the top half of the tank starts to go cold, and the system dumps excess heat (circulating at night) when the bottom half is too hot. Bottom line, the tank is half as big as the nameplate value for delivery (FHR is low), and the storage is half what you'd expect from the full volume swinging. AS it is, it stores about one day output, so your morning showers came in yesterday afternoon. If yesterday was cloudy, you are showering on electric backup heat.
So, I convinced myself between the winter months and lack of cloudy day storage, I would be unlikely to get more than 60% of my heat from the solar, the rest from electric backup. For a similar amount of out of pocket money, I can get a ASHP water heater, that will use 50% of a conventional heater (within spitting distance of the 40+% solar backup usage), get better FHR, and get a lower maintenance and more compact system. I have the perfect place for the unit, a large, semi-conditioned attached 2-car garage right next to my plumbing stack. YMMV.
So, I would love to have folks tell me I'm wrong. I'm a fan of solar and thought solar thermal would be a clear winner. But those damned refrigeration engineers are making 'free' solar BTUs look expensive.
If eff rather than $$ was the object, I would get the Velux system, leave the backup coil disconnected (getting >2x the storage) and feed it into a 50 gal geospring (to get eff backup) and then to the taps.
In 10 years all the solar tanks might have integrated HP backup....maybe I'll get one then if elec is 2-3x as expensive as now.