wrote:

Lets say it saves =A320 a year heating dhw only.

Would that be 20 UK pounds per year? Sounds like a very poor water heater.
... American families pay about $20 per month for hot water.

Welcome to uk.diy Nick. Maybe you can give them a pointer or 2 on how
to make a constructive DHW system that pays its way...

It isn't worth doing much to save 20 pounds per year, but it might make
more sense to collect both hot water and space heat from a polytunnel,
which might have a linear parabolic reflective north wall over a 2'x6'x8'
shoebox tank with an EPDM or polyethylene film liner and a $60 1"x300' piece
of pressurized polyethylene pipe for a heat exchanger and a horizontal
draindown EPDM cover with a small pump to move water up 2" over the cover
when the sun is shining, and a greywater heat exchanger, eg 2 55 gallon
drums with PE pipe in series.

Gary Reysa and I have been working on this sort of DIY stuff. See

http://BuildItSolar.com.

Nick