immersion heater running cost

[leepoll]

Hi all
Is it cheaper to leave an immersion heater constantly on or switch on to heat when needed? I would say its cheaper to leave it on rather than heat from cold, but a bloke at work reckons its cheaper to use as needed or on a timer.
Thanks in advance

 

[Howsie]

Depends on how much hot water you use. The time of day/night you switch it on(eco7). How well insulated the cylinder and pipes are.