What size hot water system do I need?

Getting the size right matters more than picking the right brand. An undersized hot water system runs out halfway through the morning routine, while an oversized one wastes energy keeping water hot that nobody uses. Terry sizes systems across Berala and Western Sydney using two different logics, depending on whether the home suits a storage tank or a continuous flow unit.

Sizing a storage tank

Storage systems hold a fixed volume of heated water, so the tank needs to cover the busiest stretch of the day. As a rule of thumb:

• 1 to 2 people: a compact tank, often 90 to 125 litres on gas or continuously boosted electric
• 2 to 3 people: 125 to 160 litres suits most homes on gas storage or daytime electric
• 2 to 3 people on off-peak electric: go bigger, commonly around 250 litres, because the tank heats overnight and must carry the whole day
• 4 to 5 people: roughly 160 to 250 litres on gas, or 250 to 315 litres on off-peak electric
• Larger households: 315 litres and up on off-peak, sized case by case

The off-peak point trips people up constantly. Off-peak tariffs only heat the tank during set overnight hours, so there is no mid-morning recovery. Whatever the household uses by evening has to fit in the cylinder, which is why off-peak tanks run a size or two larger than their gas equivalents.

Sizing a continuous flow unit

Continuous flow systems never run out, so capacity is about flow rate rather than volume. The question is how many outlets run at the same time:

• One bathroom home: 16 to 20 litres per minute
• Two or more bathrooms: 20 to 26 litres per minute

A shower draws roughly 8 to 10 litres per minute through a modern rose, so a 26 litre unit comfortably handles two showers plus a kitchen tap. One caution: manufacturers quote flow rates at a given temperature rise, and Sydney winter inlet water is cold enough to trim real-world output. Sizing to the winter figure, not the brochure headline, avoids disappointment in August.

Storage or continuous flow?

Gas availability usually decides it. Homes with natural gas and multiple bathrooms tend to suit continuous flow for the endless supply and small wall-mounted footprint. Homes without gas weigh electric storage against a heat pump, and off-peak metering shifts the maths again. Space matters too, since a 315 litre cylinder needs a solid slab and somewhere to stand, while a continuous flow unit hangs on a wall.

The mistake to avoid

Replacing like for like without thinking is the most common sizing error Terry sees. Households change. A tank that suited a couple in 2013 will frustrate a family of five in 2026, and a system sized for teenagers becomes wasteful once they move out. When in doubt, size for the busiest morning of the week, not the average one.