In this guide
Instant gas hot water systems are one of the most popular choices for Sydney households, and it's easy to see why. No tank, no standby heat loss, hot water whenever you need it. But the question most homeowners ask before committing is a practical one: how much gas will it actually use, and what will it cost me each month?
The honest answer is: it depends. But unlike other vague guides, we're going to give you real numbers, Sydney-specific context, and a clear framework so you can estimate your own usage before making a decision. We've been installing and servicing hot water systems across Sydney for over 20 years, so the figures below come from real-world experience, not guesswork.
What Is an Instant Gas Hot Water System?
An instant gas hot water system (also called a continuous flow or tankless system) heats water on demand rather than storing it in a tank. When you turn on a hot tap, cold water flows through a heat exchanger inside the unit, a gas burner fires up, and heated water is delivered to your tap within seconds.
There's no tank sitting full of water being constantly reheated. That's the key efficiency difference. Traditional storage systems burn gas continuously to keep a tank at temperature 24 hours a day, seven days a week, whether you're using hot water or not. An instant system only uses gas during the seconds or minutes you're actually drawing hot water.
Sydney note: Sydney's mains water temperature typically ranges from around 14°C in winter to 22°C in summer. This matters because the colder the incoming water, the more gas the system needs to reach your set temperature. Systems installed in Sydney's cooler inland suburbs (Penrith, Castle Hill, Campbelltown) will generally use slightly more gas than those closer to the coast during winter months.
5 Factors That Affect How Much Gas Your System Uses
No two households use the same amount of gas. Here are the five variables that make the biggest difference:
Flow Rate
Most instant systems are rated between 12 and 27 litres per minute. A 20L/min system running a large shower head and a simultaneous kitchen tap will use significantly more gas than a 16L/min unit serving a single outlet. The higher the demand, the higher the gas draw.
Temperature Rise
The difference between incoming cold water and your desired output temperature determines how hard the burner works. Sydney cold-water supply averages 14–18°C in winter. To reach 50°C, the system needs to lift the temperature by 32–36°C, drawing more gas than in the warmer months when the incoming temperature may already be 20–22°C.
Household Size and Usage Habits
A household of two adults with moderate usage is a completely different load from a family of five with multiple morning showers running at the same time. Longer showers, baths, frequent dishwashing, and multiple simultaneous outlets all push gas consumption up. Usage patterns matter as much as household size.
System Size and Star Rating
Modern gas hot water systems carry an energy star rating. A 6-star rated unit uses meaningfully less gas than an older 5-star model to deliver the same output. When upgrading, the efficiency gap between units often covers the cost difference within a few years of energy savings.
Maintenance and Condition
A heat exchanger clogged with scale or a burner running slightly rich will use more gas to deliver the same hot water output. Systems that haven't been serviced in several years often see gas consumption climb noticeably compared to a well-maintained unit. Regular servicing isn't just about reliability, it directly affects your gas bill.
Distance from the Unit to Taps
This one is overlooked but relevant in Sydney's older homes. If your hot water unit is on the opposite side of the house from your main bathroom, you're drawing water through a long pipe run. That pipe purges cold water before hot arrives, meaning the system fires up, but you're waiting and wasting. In large homes, a recirculation pump or a closer unit location can reduce this waste significantly.
What Does It Cost Per Month in Sydney?
Gas pricing in NSW varies by retailer and plan. As a rough benchmark, NSW residential natural gas is typically priced around $0.030–$0.040 per megajoule (MJ) for most households. Using the average daily consumption figures for instant gas systems, here's what typical monthly costs look like:
Important: These figures cover hot water only. If your property uses gas for cooking and heating as well, total gas bills will be higher. These estimates are based on natural gas. LPG costs are typically higher, covered in the LPG section below.
The most effective way to reduce your hot water gas costs is a combination of: choosing a correctly sized, high star-rated system; setting your temperature to 50°C rather than higher; and having the unit serviced every few years to maintain peak efficiency. Over a 15-year lifespan, a well-maintained instant system can save thousands of dollars compared to an older storage tank or an undersized system running under constant strain.
Instant vs Storage Hot Water: Full Comparison
Before committing to an instant gas system, it helps to see how it stacks up against a traditional storage tank on every metric that actually matters.
| Feature | Instant Gas System | Gas Storage Tank | Electric Storage Tank |
|---|---|---|---|
| Gas/Energy Use | On-demand only | Continuous (standby heat loss) | Continuous or off-peak |
| Monthly Running Cost | $40–$130 | $55–$140 | $70–$180 |
| Hot Water Supply | Unlimited while running | Limited to tank capacity | Limited to tank capacity |
| Space Required | Wall-mounted, compact | Large floor space | Large floor space |
| Lifespan | 15–20 years | 10–12 years | 8–12 years |
| Install Cost (approx) | Higher upfront | Moderate | Lower upfront |
| Maintenance | Low (no tank corrosion) | Moderate (anode rods, flushing) | Moderate |
| Risk of Tank Burst | None | Yes | Yes |
| Works Without Gas Connection | No (LPG option available) | No (LPG option available) | Yes |
For most Sydney households with a gas connection, an instant system is the clear winner on long-term value. The higher upfront cost is typically recovered within 3–5 years through lower running costs and the longer service life.
How to Choose the Right Size System
Getting the size right is probably the most important decision you'll make. An undersized system will leave you with cold water mid-shower. An oversized one wastes money on a unit you don't need. Here's a practical guide:
Step 1: Count Simultaneous Outlets
Think about the peak moment in your household. Are two showers ever running at the same time? Is someone showering while another person is washing dishes? Each outlet running simultaneously adds to the required flow rate. A standard shower uses roughly 9–12 litres per minute.
Step 2: Match Flow Rate to Demand
| Household Size | Simultaneous Outlets | Recommended Flow Rate | Typical Unit Size |
|---|---|---|---|
| 1–2 people | 1 outlet | 10–14 L/min | Rinnai B16, Rheem 12 |
| 2–4 people | 1–2 outlets | 16–20 L/min | Rinnai B20, Rheem 16 |
| 4–6 people | 2–3 outlets | 20–26 L/min | Rinnai Infinity 26, Rheem 24 |
| Large household / Commercial | 3+ outlets | 27 L/min+ | Multiple units or commercial grade |
SPS tip: Always factor in the coldest month of the year when sizing. A unit that just copes in summer may fall short on a cold Sydney winter morning when incoming water temperatures drop. We always recommend sizing up one step if you're on the borderline between two unit sizes.
Recommended Brands: What SPS Installs
After 20 years and thousands of hot water installations across Sydney, we've developed strong opinions on which brands hold up. Here are the three we install and stand behind:
Rinnai
Our most-recommended brand. The Infinity range covers 16 to 32 L/min, excellent build quality, strong parts availability across Sydney, and a solid warranty. Particularly good for families with medium to high demand.
Rheem
Australia's most recognisable hot water brand for good reason. Rheem instant units are reliable, well-priced, and easy to service. The 16 and 24 litre models cover most Sydney households comfortably.
Dux
A strong option for smaller households or where budget is a consideration. Good efficiency ratings, Australian-designed, and straightforward to maintain. Less common but well-supported through our network.
How Long Do Instant Gas Hot Water Systems Last?
A quality instant gas system, properly installed and maintained, will typically last 15 to 20 years. That's 5–8 years longer than a traditional storage tank system.
The reason for the longer lifespan is structural. There's no tank to rust or corrode from the inside out. The main components (heat exchanger, burner, igniter, flow sensor) are robust and individually replaceable. When something does fail, it's usually a single component rather than the whole unit needing replacement.
To get the most life from your system:
- Have it serviced every 2–3 years by a licensed gas plumber
- Flush the heat exchanger periodically if you're in a hard water area
- Check the inlet filter annually and clean it if needed
- Don't ignore error codes. Most modern units display them for a reason
- If the unit hasn't ignited or is throwing faults, call us before it becomes a bigger problem
SPS note on the hot water systems page: We noted that SPS Plumbers states the average lifespan of an instantaneous hot water heater as 12 years. With proper maintenance, the better brands regularly exceed this. Treat 12 years as the minimum to plan for, not the ceiling.
LPG vs Natural Gas: What's the Difference?
If your Sydney property doesn't have access to the natural gas network (common in some outer suburbs, rural areas, or properties that have never connected), LPG is a viable alternative.
| Factor | Natural Gas | LPG |
|---|---|---|
| Supply Method | Underground pipeline to your meter | Pressurised cylinders or bulk tank on-site |
| Availability in Sydney | Most metro and inner suburban areas | All areas, including where gas mains don't reach |
| Typical Cost per GJ | Lower (approx $30–$40/GJ) | Higher (approx $50–$80/GJ) |
| Continuous Supply | Yes, from the mains | Needs cylinder monitoring and refills |
| Environmental Impact | Lower emissions per GJ | Slightly higher emissions per GJ |
| System Compatibility | Requires natural gas-rated unit | Requires LPG-rated unit (different burner) |
If natural gas is available at your property, it's almost always the better long-term choice on cost. If you're on LPG and considering switching to natural gas, that's a worthwhile conversation to have with us, as the mains connection costs vary significantly depending on your location and how far you are from the existing network.
