“What size hot water system do I need?” As plumbers in the hot water game, you’d be amazed how often we hear that question! And it sounds like it should have a fairly straightforward answer – but hot water systems don’t all size the same way.
- A storage tank is measured in litres
- An instantaneous unit is measured in litres per minute
- A solar system is measured in a mix of panel area and backup tank size.
However, before getting into the specifics for each type, there are a few things we should work out first – since they apply no matter which system you end up choosing.
Start here: What every household needs to work out
Why start right here? Because these 3 questions apply no matter which type of system you’re sizing
1. How many people are in the household?
This is the single biggest factor in sizing any system.
A household of one or two has very different demands than a family of five, and most manufacturer sizing guides start with this number before anything else.
It’s also worth counting bathrooms, not just people, since a home with two bathrooms in regular use needs more capacity than one with a single shared bathroom, even with the same number of occupants.
2. When does everyone use hot water?
A household where showers happen back-to-back before school and work needs more capacity, or a faster recovery rate, than one where hot water use is spread evenly through the day. Two people showering at 7 am puts more strain on a system than four people showering across a two-hour window.
3. What’s your budget, and what are you connected to?
Gas, electric and solar all have different upfront and running costs, and not every property has mains gas or enough usable roof space for solar panels.
Your existing connections often narrow the field before you even start comparing tank sizes, and it’s worth factoring in running costs over the system’s lifespan – it’s not all about the price on installation day.
Have you got a rough answer to those three? The specifics for each system type should now start to make a lot more sense.
Sizing by system type
So, with the basics sorted, here’s where it gets specific to what’s actually going in your roof or cupboard. Let’s run through the sizing for each type – one by one:
What size gas hot water system do I need?
Gas storage systems are generally sized in litres:
- A 1-2 person household is suited to around 90-135L
- A 3-4 person household around 135-170L
- Larger households needing 170-400L or a continuous flow unit instead.
Hot water tank sizes for gas storage tend to sit smaller than electric for the same household, since gas reheats water faster and can keep up with demand using a smaller tank.
What size electric hot water system do I need?
Electric storage tanks generally need to be larger than gas for the same household, since electric elements reheat water more slowly.
- A single person might get by with a 125L tank
- A family of four to five often needs 250-315L or more.
If your electricity is on an off-peak tariff, sizing needs to account for the whole household’s daily use fitting into a single overnight heating window, not just total capacity, since there’s no opportunity to immediately heat up the tank again until the tariff period returns.
What size heat pump hot water system do I need?
Heat pumps are typically sized similarly to electric storage tanks in terms of litres – but their slower recovery rate matters more.
Because a heat pump draws heat from the surrounding air rather than generating it directly, it takes longer to reheat a depleted tank, so it’s worth sizing slightly larger than you might for an equivalent gas system to avoid running short during high-use periods.
What size solar hot water system do I need?
Solar systems are sized on two fronts at once:
- The number of collector panels or tubes needed to capture enough energy for your household
- The size of the storage tank that holds the heated water and backup supply.
A 1-2 person household typically needs one or two panels and a smaller tank, while a larger family may need three or more panels paired with a 300-plus L tank to cover cloudy days when the booster does more of the work.
Roof orientation and pitch also affect how much a given panel count can realistically deliver, which is why two homes with the same household size can still end up with different-sized solar setups.
What size instant hot water system do I need?
Instantaneous units aren’t sized in litres at all, since there’s no tank to fill. Instead, sizing comes down to flow rate – measured in litres per minute – and how many hot water outlets might run at once.
- A single bathroom might only need a unit rated around 16-20L/min
- A home running a shower and kitchen tap simultaneously needs a higher-rated unit, or two units installed in parallel, to avoid a drop in water pressure or temperature.
Getting it exactly right for your home
Figuring out what size hot water system do I need Australia wide isn’t really about finding a single national answer – since climate, household habits and what’s already connected to your property all move the numbers around quite a bit. A Newcastle household on mains gas will land on a different answer than a Central Coast home with good roof exposure for solar – even with the same number of people living there.
Rather than guess, Fluid Plumbing can assess:
- Your household size
- Your usage patterns
- Your existing connections on-site.
Then we’ll recommend a system sized correctly for your specific home. Get in touch with Fluid Plumbing, and we’ll work out exactly what your household needs before anything gets installed.
