Why?
A 300 litre hot water cylinder requires 15kWhrs of energy to heat it.
Assuming electricity is 25c per kWhr, the cost per day is $4 (to make things easy).
A heat pump will save 10 kWhrs a day (more in summer, less in winter), hence each cylinder of hot water will represent a saving of $2.50.
For a complete year, that's over $900.
Many families will use a cylinder of hot water each day.