To get the right sized tank:
1. Count the number of people in your household and consider the people who might join or leave your household soon.
2. Assume that each person will use around 50 litres of hot water per day. (Modify if someone prefers cold showers or very long, very hot ones).
3. Require that your hot water tank keeps 1.5 days’ capacity at all times.
4. Thus, multiply the number of people by 75.
Even then, most heat pumps can cope with larger households because they have a timer which can be set to when you want the HWHP to turn on. Also, most HWHPs have a “Boost” button. It is not recommended to connect the HWHP to a dedicated off-peak electricity rate for larger households.