Does leaving hot water tank on waste electricity?

I have a hot water tank in a cupboard. There is a switch which you have to press in order to heat the water. At present I leave the switch on so I have constant hot water. Does this waste electricty and if so roughly how much? Or would it be sensible to only switch on when I need hot water?
Update: MESSAGE TO TONY BRAD. THIS IS THE FIRST TIME I HAVE ASKED THIS QUESTION. I HAVE NEVER COMMUNICATED WITH YOU BEFORE LET ALONE RIDICULED ME. I AM NOT BLIND.
Update 2: RIDICULED 'YOU' I MEAN.
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