Solar hot water: how it works, and why the booster decides everything
The collectors on the roof are the part you can see. The part that determines whether the system saves you anything is the booster in the tank, and the mode it is left in.
A solar hot water system does not heat your water on its own. Every one sold in Australia has a booster — gas or electric — and on a wet week in July the booster is doing most of the work. That single fact explains almost every complaint people have about solar hot water, and it is the thing brochures put in the smallest type. This page is an independent explanation of how the systems are laid out, what the booster really does, and what to ask before you commit; we sell nothing and have no relationship with any manufacturer.
How does a solar hot water system actually work?
Two parts do the work. On the roof, a collector — either flat panels or evacuated tubes — absorbs sunlight and heats either the water itself or a fluid that carries the heat. In a tank, that heat is stored until someone opens a tap. The sun is free and unreliable, so a booster element or gas burner tops the tank up whenever the sun has not got it hot enough.
The layouts differ in where the tank sits, and that is a decision you make once and live with for twenty years.
| Layout | Where the tank is | What it suits | The trade-off |
|---|---|---|---|
| Close-coupled (thermosiphon) | On the roof, directly above the collectors | Simple installs, no pump to fail, warm climates | Serious weight on the roof structure, and the tank is visible from the street |
| Split system (pumped) | On the ground, with a pump moving fluid to the roof | Roofs that cannot take the load, or where you want the tank out of sight | A pump and controller that can fail, and a little standby power |
| Evacuated tube | Either layout — the difference is the collector | Cooler and cloudier climates; tubes lose less heat to cold air | Individual tubes can be damaged; usually a higher outlay |
| Heat pump (often sold alongside) | Ground level, no roof collector at all | Shaded blocks, units, heritage overlays, anywhere a roof array is impossible | Not solar at all — it is an efficient electric system. Noise and clearances matter |
What is the booster, and why does it decide everything?
The booster is the backup heat source that guarantees hot water regardless of weather. It is either an electric element in the tank or a gas burner, and it is not an optional extra: a system quoted without one is not a complete system.
What matters is how the booster is controlled, because that is what separates a solar system that saves you money from one that quietly behaves like an ordinary water heater with panels on top:
- Manual or switched boost. You turn it on when you need it. The most economical setting, and the one households most often forget about before complaining of cold showers.
- Off-peak or controlled-load boost. The element only runs in a set overnight window. Cheap to run, but if the tank is undersized you can empty it in the evening and wait until the small hours.
- Continuous boost. The element maintains temperature around the clock. It never runs out, and it also means that through a cloudy fortnight you are running a conventional electric tank at conventional electric running costs.
- Gas boost. Fires only on demand and recovers quickly, which is why it is common where natural gas is already at the meter.
The honest summary: a solar hot water system performs as well as its boosting strategy allows. Ask the installer which mode yours will be set to, and ask them to show you where the switch is.
Does solar hot water work in winter?
It works, but it contributes less, and how much less depends on where you live far more than on which brand you buy. A collector in Cairns and the same collector in Hobart are doing different jobs. Two things fall at once in winter: there is less sun, and the mains water arriving at the tank is colder, so more energy is needed to lift it to temperature. That is why the winter months are when the booster earns its place.
Frost is the other winter question, and it is a real one anywhere inland or at altitude. Systems sold for frost-prone areas use a closed circuit with a heat-transfer fluid, or a frost-protection routine, rather than running mains water through exposed roof pipework. If you are anywhere that gets frosts, ask specifically what the frost protection is and make sure the answer appears on the quote.
Is my roof suitable?
Four things decide it, and an installer should be checking all four before quoting:
- Aspect. In Australia the collectors want to face roughly north. East or west is workable with more collector area; a roof with only a southern face is a serious problem.
- Shading. Not just now — a neighbouring tree in ten years, or the block behind you being developed. Morning shade matters less than midday shade.
- Structure. A close-coupled system puts a full tank of water on your roof. That is a structural question, not a plumbing preference, and it is the usual reason a split system gets recommended instead.
- Pitch and available area. Enough uninterrupted area, clear of vents, whirlybirds and the solar PV array if you already have one.
If the roof rules it out, the conversation usually turns to a heat pump, which does a similar job with no roof hardware at all. That comparison is worth having properly — see heat pump hot water and the wider comparison of all four types.
Solar thermal is not the same as solar PV
This confuses more conversations than any other point in the category. Solar thermal heats water directly with sunlight, using collectors plumbed into a tank. Solar PV generates electricity, which can then run any electric appliance — including a heat pump, or a resistance element in an ordinary tank.
Both are legitimate ways to heat water with the sun, and they behave very differently. A PV-plus-heat-pump setup is more flexible, because the same panels also run everything else in the house. A thermal system is more directly efficient at the one job it does. Which is better for you is a real question, and anyone who answers it instantly without asking about your roof, your existing PV or your household is selling rather than advising.
What a proper solar installation includes
Whichever layout you choose, these belong in the job and are worth confirming in writing before work starts:
- A tempering valve where required, limiting the temperature delivered to bathroom outlets. Solar tanks can reach high temperatures on a good day, which makes this more important here, not less.
- A relief valve with a discharge pipe running to a safe point, correctly sized and not terminating where it can scald someone.
- An isolation valve you can actually reach.
- Frost protection appropriate to your climate, stated explicitly.
- The booster type and control mode, and where its switch is.
- A compliance certificate for the work. Keep it with the property records — see licensed work.
- For a split system, where the pump and controller sit, and how you would know if they had failed.
Where you stop and a licensed plumber starts
Installing, replacing or relocating a solar hot water system is licensed plumbing work in every Australian state and territory, and where a gas booster is involved it is licensed gasfitting as well. That is not a recommendation on this site's part; it is how the trade is regulated, and each state and territory administers it through its own body.
What you can reasonably do is observe and report: notice that the water is cooler than it was, that the relief valve is dripping steadily, that the roof tank is leaving a stain on the roof sheeting, or that the booster seems to be running constantly. All of that is genuinely useful information to hand a plumber. What you should not do is go onto the roof, open the tank, or touch anything inside the electrical enclosure of a booster element. If your system has a gas booster, nothing on the gas side is a homeowner job — not relighting it, not adjusting it, not inspecting the burner.
If the reason you are reading this is that an existing system has stopped delivering hot water, start with the fault rather than the replacement: see no hot water, what to check in order. Replacing a system that only needed a part is the most expensive mistake in this whole category, and on a solar system the part is very often the booster rather than anything on the roof.
Frequently asked questions
Does a solar hot water system need a booster?
Yes. Every solar thermal system sold in Australia has a gas or electric booster for cloudy stretches, winter and heavy use. A quote that does not include one is not a complete system. How the booster is controlled — manual, off-peak, continuous or gas — is what decides whether the system behaves like a solar system or like an ordinary water heater with panels on the roof.
Does solar hot water work in winter?
It works but contributes less, because there is less sun and the incoming mains water is colder, so more energy is needed to reach temperature. The booster covers the shortfall. In frost-prone areas the system needs specific frost protection, usually a closed circuit with a heat-transfer fluid — ask for it to be stated on the quote.
What is the difference between solar hot water and solar panels?
Solar thermal heats water directly, using roof collectors plumbed into a tank. Solar PV generates electricity, which can run any appliance including a heat pump or an element in an ordinary tank. Both heat water with the sun; they are different systems with different trade-offs, and a PV array already on the roof changes which one makes sense.
Can the hot water tank go on the ground instead of the roof?
Yes — that is a split system, where a pump circulates fluid between the roof collectors and a ground-level tank. It is the usual answer when the roof cannot carry the weight of a full tank, or when you do not want it visible from the street. The trade-off is a pump and controller that can eventually fail.
Can I install a solar hot water system myself?
No. Installing, replacing or relocating a hot water system is licensed plumbing work in every Australian state and territory, and a gas booster makes it licensed gasfitting as well. Each state and territory regulates it through its own body. What you can do is observe and report symptoms; roof work, the tank and the booster's electrical enclosure are not homeowner territory.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NSW Climate and Energy Action. Solar water heaters
- Solar Victoria. Section 3: Choosing between a heat pump and solar hot water system
- Clean Energy Council. Solar water heating
- NSW Fair Trading. Compliance and regulation — plumbers, drainers and gasfitters