I'm working on a project in Rhode Island (I posted about this same project a while ago - thanks for those responses!) To recap, this is essentially a summer only toilet and shower facility for a small (40 -50) boat dock. The facility is open june through september then shutdown completely. I'm thinking of doing a very simple batch type system by installing glazing in the south facing roof (roof pitch at latitude of 41 or a bit shallower) then installing a horizontal black tank under the glazing and insulating the area around it heavily and with some fairly reflective material (foil faced drywall perhaps). The system would work as a preheat for the shower demand (which I can't seem to get data on, but is large - I'm told folks line up for showers all morning- so four showers going fairly constantly for the morning hours. I don't expect to meet all of the demand and back up will be a propane demand heater installed between the solar tank and the showers. I would like to present to the owner (which happens to be a town, not a private owner) some realistic figures as to what this system might contribute. This project is on an island so utility costs are very high - making paybacks much shorter. The Owner understands this but would still like to get some reasonable numbers as to good an idea this is and what saving might accrue which is certainly a reasonable request. What I really need to do that, is some source for what solar radiation, in BtU's per unit area, I can expect on a 41 degree south facing surface at various dates in June through September. I managed to find some data for vertical windows but that includes ground reflection and window performance and I'm not sure how to translate that into a sloped surface. A couple questions - if I find data for kWh/meter2 on a flat surface is that directly "translatable" into Btu/ft2 by using a conversion of 3412 BtU/kWh and then doing the geometry to translate the flat surface into the sloped surface relative to the sun angle? The conversion meter2 to foot2 is obvious but the slope question seems trickier - is there a "reflection factor" typically built into some of the raw data for insolation? Also, with this system should I be concerned about collecting heat all day then reradiating it back out at night? Since the demand is in the morning hours I'll essentially be collecting one day for use the next morning. Should I think about some sort of operable cover for the night? Many thanks. Great Forum!!! Sam Any help would be appreciated!!

Check out Gary Resya's Site BuildItSolar.

The Solar Site Survey seems to have links to most of the resources you need but he also has efficiency calculations for panels that he's build and probably others - there's a load of information on the site.  The solar data includes data for various collector angles.  I'm not sure that you have the optimum angle for summer collection, which is all you're interested in.  I think that latitude is a compromise for year-round collection.

I'm no design expert, but it would seem that you need some significant storage somewhere since your demand is before you'll get any significant solar help in the morning.  I think that you'll need a closed system and a heat exchangers since I'm not sure that you should be pushing the shower water through the panels for potential health reasons, but I wouldn't take my word for that.

If you're not pumping at night you shouldn't loose much heat.

It should be a good project since you're looking to heat the water when you have the most sunlight, the opposite of most systems, especially space heating systems.