What Size Hot Water System Do I Need? A Sizing Guide for Sydney Families

Get the size right and you never think about your hot water again. Get it wrong and you either run cold halfway through the third shower every morning, or you pay to heat water nobody uses for the next decade. Here’s how to size it properly, for storage tanks, continuous flow, and heat pumps.

First: two different systems, two different ways of sizing

Storage systems (a tank of hot water) are sized in litres: the question is how much hot water your household uses in its peak period, usually the morning rush.

Continuous flow systems (heat water on demand, no tank) are sized in litres per minute: the question is how many hot outlets run at the same time. They never “run out,” but an undersized unit can’t feed two showers and a kitchen tap simultaneously.

Storage tank sizing by household

As a working guide, assuming typical usage of roughly 50 litres of hot water per person per day, with mornings as the peak:

Household	Electric storage (off-peak)	Gas storage	Heat pump
1 to 2 people	125 to 250L	90 to 130L	160 to 215L
3 to 4 people	250 to 315L	130 to 170L	250 to 300L
5+ people	315 to 400L	170L+ or continuous flow	300 to 400L

Why electric tanks need to be bigger than gas: most electric storage systems heat overnight on off-peak tariffs, so the tank must hold a full day’s hot water. Gas storage reheats quickly through the day, so a smaller tank keeps up. This is the single most common sizing confusion we see: the litres aren’t comparable across fuel types.

Continuous flow sizing by bathrooms

Home	Typical size
1 bathroom, 1 to 2 people	16 to 20 L/min
2 bathrooms	20 to 26 L/min
3+ bathrooms / large family	26 to 32 L/min, or two units

Rule of thumb: a shower draws roughly 7 to 9 litres of hot water a minute with a water-efficient head. If two showers and the kitchen sink can plausibly run at once in your house, and with teenagers, they will, size for that moment, not the average.

The questions that change the answer

• Do you have a bath you actually use? A big soaker tub can drain a small tank on its own.
• Teenagers now, or soon? Shower duration roughly doubles. Size for the household you’ll have in five years, not today’s.
• Gas available at the property? If not, the realistic contest is electric storage vs heat pump, and with current NSW rebates, the heat pump usually wins on ten-year cost. See our full guide to hot water replacement costs and NSW rebates.
• Where will it live? Heat pumps need airflow and a spot where low compressor noise won’t bother a bedroom. Continuous flow units free up the space a tank occupied.
• Solar panels on the roof? A heat pump on a daytime timer effectively runs on your own solar, one of the best pairings in home energy right now.

The oversizing trap: “just go bigger to be safe” costs real money on storage systems. You pay to heat and reheat water you never use, every day, for the life of the unit. Bigger is not safer; correctly sized is safer.

Replacing like-for-like? Check before you assume

If your current system never ran out and never felt wasteful, matching its size is a sensible starting point. But if you’re replacing a system installed by a previous owner, don’t assume they got it right, and if your household has changed since (kids arrived, kids left, bathroom added), resize from scratch. A replacement is also the one moment switching system types costs the least, since you’re paying for installation anyway. Our hot water team can size and quote options on the spot.

Frequently Asked Questions

What size hot water system for a family of 4?

Typically a 250 to 315L electric storage tank, a 130 to 170L gas storage tank, a 250 to 300L heat pump, or a 20 to 26 L/min continuous flow system. The right choice depends on your fuel type, peak usage and bathrooms.

Is it better to oversize a hot water system?

Not with storage systems. An oversized tank continuously heats water you don’t use, wasting energy every day. Continuous flow systems are more forgiving, since they only heat water on demand.

Can a continuous flow system run two showers at once?

Yes, if it’s sized for it. A 26 L/min unit comfortably runs two water-efficient showers simultaneously. A 16 L/min unit generally can’t without a noticeable drop.

How much hot water does one person use per day?

Around 50 litres per person per day is the standard planning figure, though long showers, baths and teenagers push this up significantly.

Need a hand? Call The Plumbing Paramedics on 0430 054 007.