Thats interesting uberalice...DH thinks (not entirely sure where he gets the info from) the solar water heating is less cost efficient. You have obviosuly done the sums and decided that its a better option. Do you have a south facing roof though?
Not quite south facing, but our upstairs gets very warm during the summer, so the roof is clearly getting a lot of sun. I'll get you some more information later on (sorry, I promised that last night and then went away).