Water heater

Too broad a question.

Factors like tank size and insulation and potential usage levels all come into play.

We have a holiday apartment with the tank in a kitchen cupboard, it doesn't appear to be insulated. There would be 2 or 3 adults staying for short breaks and all showering first thing in the morning and possibly again later if going out. We've been putting it on overnight but I read somewhere that it's better to leave it on? 

You'll get mixed opinions on that.but few based on anything other than guesswork, conjecture, or worse, bar stool 'expertise', and of course once again it can depend very much on individual usage patterns, there is no 'one size fits all' answer.

The only way to conclusively prove what works most economically for you personally is to run it through an energy meter and over a reasonable period of time actually track how much electricity it consumes in each mode.

In the home I'm having built I've opted for solar water heating backed up with an integral immersion heater for if or when necessary and that's exactly what I'll be doing but I'm reckoning on it taking me many months of experimenting and monitoring to reach a conclusion as to which works best for me in the long term.

We left ours on all the time as you never know when you will need hot water.  If you are on Iberdrola go to your nearest office with a bill and passport of whoever is named on the bill and ask about the night time tariffs this will put your electric on less than half price electricity for 14 hours  22.00  till 12 noon winter and 23.00 till 13.00 summer this is also ideal for air con units, washing in the morning etc 

That illustrates the comment I made in my earlier post about varied opinions.

If you have a reasonably modern storage tank then water heated overnight should remain hot enough for the whole day.

In our previous place in France we had fairly new 200lt tank which was supplied with power in the small hours via a simple mechanical timer but the water was still hot enough in the evenings for showers or a bath.

One year we had no less than 8 people in the house yet not once did we run out !