What Size Hot Water System Do I Need?

Getting the size right matters more than the brand on the badge. Undersize a hot water system and someone’s shower runs cold every morning; oversize it and you pay every day to keep water hot that nobody uses. Here’s how we size systems for homes across the Northern Beaches and North Shore.

Storage tanks: size by household

Storage systems (electric, gas or heat pump) hold a fixed amount of hot water, so they’re sized to the people using it:

Household	Electric storage (continuous)	Electric off-peak	Gas storage
1–2 people	125–160 L	160–250 L	135 L
3–4 people	250–315 L	315 L	135–170 L
5+ people	315 L+	400 L	170 L+

Two adjustments worth making:

• Off-peak tariffs need bigger tanks: the tank reheats only overnight, so it must store the whole day’s usage.
• Gas storage tanks run smaller than electric for the same household because they reheat much faster during the day.

Heat pumps follow storage sizing: 270–315 L covers most family homes.

Continuous flow: size by simultaneous use

Instantaneous units never run out of hot water, so household size doesn’t matter: how many outlets run at once does. Sizing is in litres per minute:

Bathrooms	Typical unit size
1	16–20 L/min
2	26 L/min
3+	32 L/min, or twin units

The classic mistake is fitting a small unit to a two-bathroom home: it works fine until the dishwasher, one shower and a basin tap all run at once, then everyone gets lukewarm water. The second, sneakier constraint is the gas line: bigger units draw more gas than older lines were sized for, so we check supply before quoting, not after installing.

The mistakes we see most

1. Like-for-like replacement of an undersized system. The old tank was struggling for years, and the new identical one struggles from day one. Replacement day is the cheapest moment to fix sizing.
2. Sizing continuous flow by household instead of outlets. Five people and one bathroom needs a smaller unit than two people and three bathrooms.
3. Ignoring recovery time. Electric storage reheats slowly; if your household clusters its showers in one hour, recovery matters as much as capacity.
4. Forgetting the future. Kids get older and showers get longer. If you’re between sizes, go up one.

Get it sized by someone who’ll stand behind it

Tell us your household size, bathrooms, and what you have now, and we’ll size it properly and quote the full job. If the system’s already down, our 24/7 repair team can usually restore hot water the same day. And before you choose fuel type, see gas vs heat pump running costs.