If you can do the maths you will find its always cheaper to turn it off
and
reheat unless you have a very odd tariff.

I think that is an interesting point, and may be behind the "immersion
heater" story. With a well insulated tank, and a policy of taking a large
bath every evening, and on "Economy 7", you might a disbenefit in switching
off for part of the "cheap energy" window.