I am getting ready to build and I am looking at evacuated tube solar for hot water heating. I have looked at several different manufacturers and they give a total Btu per day collected for different scenarios. I am looking for a Btu/hr collected number? I can calculate based on water flow and inlet temp what the final temp will be. Whould think this is a common number that most sullpiers would have but can not find it. thanks. Eric

You would need to know the daily irradiance period (in hours) for the associated daily situations to do the calculation. We have software on our website to the determine the daily irradiance period and associated clear sky and climatic heat gain for passive solar building, but it includes the effects of having a roof overhang, fenestration area/SHGC, and local terrain obstructions. You might want to check the Build-it-Solar website for info on this subject.

We don't typically care too much about an hourly production because the suns output is all over the map. Rated outputs are based on very specific test criteria, ie: 1000w/ m2 of surface area or aperture area at 25C. We know from experience that a good panel will have a range of appropriate flow rates throughout the day and this is usually between 0.4 to 0.75L/m2 of aperture area (which is easier to state for flat panels than vacuum panels) and that most good controls now have variable speed pumping that will maximize the thermal output based on the supply and return temps and the load temp.

Depending on what you want to do, a vacuum tube may not be the right panel for you. If you are in the southern part of the USA, the chances are that a flat panel will be just as good doing DHW as a vacuum panel and with less maintenance. If you have a lot of sub zero temps, the vacuum panel will give more output, annually. If you want to oversize the system for space heating, you MUST make sure to have a heat dump for excess summertime heat or the glycol and tubing life will be much shorter.

There are very good free solar simulation programs out there. Look up Polysun or T-Sol. I use both of the professional versions of these.

What Mike said... Because of the difference in the ammount of atmosphere the suns rays must pass thru from sunrise to sunset at your site(least amount at local noon), The suns energy reaching the surface of the earth at a specific site varies steadilly throughout the day as a fixed panel sees it. So a specific BTU/HR output is a very difficult number to advertise reliably. The only one I have ever seen was for a tracking solar parabolic reflector and it was based on sq/ft of reflector area, but I am sure it also varied from early morning to late efternoon. With a fixed collector, that total BTU per day figure is delivered over a bell curve with the highest part of the curve lasting over 4-5 hours centered at local noon. You could spread that total BTU/day figure for a given weather/season condition over a bell curve to get a rough idea of hourly output, but because solar is all over the road, it is rarely calculated like this. Since it typically comes down to how much heat will this give me on a sunny day, or a partly cloudy one, that is the most logical way to express it.
If this is a primary heat source, well you want as much as you can get and a large storage tank and collect it when it is available and store it for use when the sun is unavailable. If it is a supplement, well you size it to provide most of your input when available, usually guaged by ammount of reduction in other energy consumed...