ER> The existing system in the Mobil is 20 - 25 gallons with a 40
  > gallon backup Solar System, should give us more than enough
  > water, we think!
ER> Can't thank you enough for all this help. Really appreciate it.
I am pleased to be of service. 
Since you live in Canada, I expect your winter days are a bit
shorter, and a lot cooler in the winter time. I think a 40-gal tank is
reasonable, but you will need to include features to aid in energy
retention.
Make sure that the talk and box is perpendicular to the winter angle
of the sun. Make sure it gets the maximun sun time. Five hours of sun
in the winter time will not work very well. Summer would be okay.
I strongly reccommend insulating the back, sides, top and bottom of
the box with R-13 insulation, minimum; and line the inside with foil
to reflect the heat towards (and back to) the tank. Also, use dual
pane glazing. It's important that you keep the heat from radiating
back out once you captured it.
Also, becuase heat losses during those cold winter nights can be
substantial, I strongly suggest an insulated cover. It needs to be
opened whenever the sun shines, and closed any other time. This is a
more complicated part of the project, and you can leave it off and
study the performance first. But I think you will need it when winter
comes.
An insulated cover need only be a simple wood frame sandwiching some
insulation. Making it with quarter-inch plywood, 2x4s and R-13
insulation will work fine. If you hinge it on the bottom, it can be
drawn up like a draw-bridge. That would be the simplest way to do it.
I don't have a cover. I haven't tested my latest design during the
winter. Last winter I merely had plastic sheeting, and it would only
last a few months before the sun deteriorated it and the wind tore it
up. I have since installed 'sun resistent' plastic sheeting like the
kind you see in green houses. It is more forgiving and a little less
expensive than glass - and safer to work with.
The design I want to use it to have two halfs, and hinge it on the
sides. With the inside cladded with foil, they would serve as extended
reflecters for added heat collection. Like a parabolic dish. Granted,
automating the covers so they would open and seek the angle to reflect
the sun to the tank would require a bit of technology, but I think it
is doable.
Good luck, and keep us posted on your progress.
--- Maximus/2 3.01
---------------