How much Power for a Shower?
According to the energywise website it costs around 33c for a 5 minute shower. So if you have 5 people in the flat taking long 15 minute showers it will cost about $5 per day.
So perhaps the easiest way to save hot water is to put a clock by the shower. If each person cut just 1 minute off their shower time you could run the flat heatpump for 1 hour.
In NZ, winter hot water energy use increases by about 60% due to longer showers and having to heat water with a colder starting temperature.
For the average NZ home around 30% of the electricity is used for heating water. About another 30% is used for space heating (if electric heating is the heating source).
Often we find that student flats are not heated as much as the average NZ house, so if your flat is conservative with space heating then water heating and long showers may be closer to 50% of your electricity bill.