Hot Water System Sizes: What Size Do I Need?

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Oh-oh: Did your steaming-hot shower just go stone cold for the third time this week? Has your family gotten older and bigger, teenagers are having longer showers than ever, or perhaps the kids have moved out and you don’t need to replace your old system with such a big unit? There are plenty of reasons why you might be Googling for a hot water system size calculator – and getting no joy.

Well, we’re here to help, because we understand better than most that once you’ve decided what hot water system technology-type works best for you (tankless or tank, electric or gas hot water system, etc.), you then discover you need to choose between all those hot water system sizes! “What size hot water system do I need?” Right here, and right now, let’s get into how to make the right decision for your home, space requirements, family size and age, budget & lifestyle.

At Fawcett Plumbing, one of the best and most experienced Adelaide plumbing companies, we get asked about hot water tank sizes each and every day. And that’s no surprise because storage tanks range in size from teeny-tiny 80-litre ones to big-Daddy 400L+ tanks. Even for instantaneous systems, the most popular units differ wildly in litres-per-minute capacity – and the tag price and the ongoing cost of running all of these systems differ just as wildly.

The bottom line is that you need to make a wise choice because if you go too small, you’re going to overwork the unit or keep running out of water. Too big, and you’re just ramping up your bill, emptying your wallet and wasting energy. Therefore, perhaps more than most things in life, you really do need to get it just right.

How to achieve that perfect balance? Start by asking yourself these questions:

1. How big is your family?

It goes without saying, but a family of 7 is going to use more hot water than a bachelor. But before you decide on a size for today, ask yourself if you have plans to stay in the house and start a family tomorrow, because hot water system life span and warranties run for as long as a decade or more.

2. How many bathrooms do you have?

Again, there’s a major difference between a small flat and a whopping great house with three bathrooms and perhaps a couple of kitchen sinks, too – and some people even prefer washing machines that use hot water. If your home or premises is going to be using more than 1 hot water outlet at the same time, that’s another major factor to consider.

3. How much water do you use?

Think about when most families crank up their hot water use: it’s in the evening, after school and work. That can place a lot of demand on the system you choose, overwhelming its ability to keep up or catch up with hot water production. On the other hand, for a similarly-sized family that uses water more evenly throughout the day, they can get away with a smaller system.

4. Is off-peak an issue?

In Adelaide but also elsewhere in Australia, there are peak and off-peak pricing tariffs for power that really can affect what size hot water system you need. Off-peak means it’s significantly cheaper to heat a full tank of hot water overnight, so if you get a tank big enough, you can heat an entire day’s water for the lowest possible price. But for non-tank technology or non-electricity energy, that’s not a factor.

With all of that in mind, you’re ready to get into the raw numbers that should guide you towards choosing the hot water tank sizes that work for you:

Off-peak electric hot water system (tank):
1-3 people – 250L
3-5 people – 315L
5-8 people – 400L

Peak electric hot water system (tank):

1 person – 25L
1-2 people – 50L
2-3 people – 80L
3-5 people – 125L


Instantaneous gas hot water systems (tankless):

1 outlet – 9L
2 outlets – 18L
2-3 outlets – 27L
More than 3 outlets – 36L


Do you still need help with your hot water system sizes?

At Fawcett Plumbing, we understand that for the vast majority of people, installing a new hot water system is a major decision and investment that you’re stuck with for years and years – so you really can’t afford to get it wrong. That’s why we’re more than happy to help you match the perfect system with your home, your habits and your budget. We don’t just repair and install hot water systems, for the best possible choice, we also offer free and friendly advice.

Whether you’re in Athelstone, Campbelltown, Magill, Hope Valley, Prospect or anywhere in Adelaide, our plumbers are always ready to help with all kinds of hot water issues  – so don’t hesitate to give us a call today.