What is cheaper to run the hot water through the timer or the immersion switch?

We have a hot water timer which also controls the central heating. The water comes on three times a day and for an hour at each interval which is enough for our household. Recently a friend said it would be cheaper to run the hot water off the immersion switch as once it reaches temperture it cuts out and you can leave it on all day and it will use less energy. Is this true that it woulk be cheaper to run the immersion switch constantly or would it work out better on the wallet if we stuck to our three times a day timed water?