Hi Floyd
FW> Anyone have the coefficient for the rate of heat absorption per
  > square foot of black surface exposed to the sun? Or even better:
  > how big does a passive solar water heater have to be (a
  > practicable size and rate of heating) to supply 100 gallon of hot
  > water per day.
I need more information on the last part of your question. If you
paint your tanks ultra-black, and you don't live higher than 60-deg
lat, then you can expect about 100W/per sq/ft. input.
I used three water heater tanks in a 5x8 box - that's 40 sq/ft. I live
in a moderately sunny area, winter excepted. For about three months I
get no hot water, for three months I get no hot, but warm enough for
bathing water; and for six months I get enough hot water (140gals) to
meet our needs for a family of four.
Right now in the afternoons the water too much too hot to touch.
That's when we run the dishwasher. Those who adjust their lifestyle
just a little will save tons of bucks. We are on propane, it use to
cost $200 a year. Not even half that now.
 
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