"Neil Jones" wrote in message
...
Should the cylinder be switched off Monday - Fri am allowing the water
to
cool and reheat from what I asume would be almost cold on Friday pm.
Or, is
it more energy efficient to leave it on and let the thormostat
regulate
power consumption to keep it up to temp throughout the week?

Turn it off. It probably won't be almost cold - my brother has a 150l
cylinder in his flat which was switched off last Monday and on Saturday
the water was still warm.

What cylinder is this ? Our cylinder goes cold in 2 to 3 days. It also
costs about 10p/day just to keep the water warm without using any. Even
tried adding a standard cylinder jacker over the spray foam but that didn't
make a major difference - temperature between the jacket and the foam was
about 2 to 3 deg above ambient.