In an extremely vague broad measurement of Domestic Hot Water consumption, the average house was found to use about 120 litres. About 4% of houses consumed more than 300 litres.
The Hot Water Association says that, for high consumption, one should allow up to 70 litres of water per person.
Imagine and large and full house with 6 people, each consuming 70 litres of hot water per day. That is 420 litres (420 kg) of hot water per day.
Hot water in the home typically requires temperature increases of 40 K to get it from averagely cold (10 °C) to averagely hot (50 °C).
So energy needed to heat all this hot water
= 420 kg x 4.2 J/kg K x 40 K = 70,560 kJ in 24 hours.
24 hours = 24 x 3600 seconds = 86,400 seconds.
The average power needed to heat our extravagant hot water system is therefore:
70,560 kJ / 86,400 s = 0.82 kW.
Therefore, if you have an extravagantly large water consumption, in a pretty full house, you will still only need an average power of less than 1 kW to heat all that hot water.
Typical boiler powers for an average house are over 20 kW. Compared with boiler power, the power needed for hot water is trivial. Therefore, if you get your hot water system and storage right, you need to allow only an extra kilowatt, even for extravagant hot water provision.