Bob,I have been thinking on the same lines as you,I have a Combi boiler and have it now on "water only" but also have an immersion tank and was wondering ,like you,if it would be cheaper to use that...of course,one has to heat up the whole tank but once you have done that,it would not take much to top it up,my father was an electrician and he told me not to just put it on when one wants hot water as then it will take longer as it will have cooled down(do have a jacket on the tank)....to keep it switched on.That was the days before central heating ..