What size water heater or cylinder do you need?

Size for the peak, not the day

What matters is the busiest stretch: how many showers run back to back, or at the same time, before the heater has had a chance to recover. A family of five who all shower in the hour before school and work needs much more stored hot water than five people spread across the day.

Shower type changes everything. A rain shower or body jets can use two or three times the water of a standard head, and filling a bathtub takes a large share of a small heater in one go.

Recovery: storage is only half the answer

A heater that recovers quickly can be smaller. A cylinder heated by a powerful boiler reheats much faster than one with a single electric element, so it can serve the same family with less storage.

Hot water is also stored hotter than you use it. Water kept at 60 °C is mixed with cold at the tap, so each litre in the tank makes more than a litre of shower water. That is one more reason not to turn the heater down.

Rough starting points

For a single electric storage heater serving one bathroom with standard showers, 50 litres is a common starting point for one or two people, 80 to 100 litres for three or four, and 120 litres or more for larger families or where there is a bathtub. Treat these only as a starting point.

For a central cylinder serving the whole home, the calculation is different: it adds up the bathrooms that can be used at once, the shower types, any bathtubs and the recovery of the heat source. That is the job of a proper design, not a rule of thumb.

Why bigger is not always better

An oversized tank loses more heat while it waits, takes longer to heat from cold, and leaves water standing unused, which is exactly what Legionella likes. A holiday home used only at weekends is a common case where a smaller, faster heater, or a central system with good controls, works better than a very large tank.