Solar hot water systems are one of the most cost-effective and energy-efficient ways to heat water in Australia. With high solar exposure, generous rebates and increasingly expensive electricity, understanding how a solar hot water system works helps homeowners choose the right design, troubleshoot issues, and maximise long-term savings.
This guide breaks down solar hot water in clear Aussie English — from collectors to tanks, boosters, frost protection and real performance expectations in each climate zone.
Key Takeaways
- Solar hot water uses roof collectors to absorb sunlight and transfer heat to a storage tank.
- Systems produce 50–90% of yearly hot water energy from the sun depending on climate and setup.
- Two main designs in Australia:
✔ Thermosiphon (tank on roof)
✔ Split-system with pump (tank on ground)
- Two main collector types:
✔ Flat-plate collectors
✔ Evacuated tube collectors
- All systems need a booster (gas or electric) for cloudy days and winter.
- Solar hot water qualifies for STC solar rebates, reducing upfront cost significantly.
- Best suited for homes with good north-facing roof space and moderate to high hot water usage.
1. What a Solar Hot Water System Actually Does
A solar hot water system heats incoming cold water using solar radiation instead of electricity or gas. The system consists of:
- Solar collectors (roof panels)
- Storage tank (roof or ground-mounted)
- Booster element or gas burner
- Pump (in split-systems)
- Temperature sensors and controllers
- Frost protection devices
The goal is to deliver water stored at 60°C+ to a tempering valve, which reduces it to a safe 50°C bathroom temperature.
2. How Solar Hot Water Collectors Work
Flat-Plate Collectors (Most Common)
Imagine a dark-coloured metal box with a glass cover.
Inside are:
- A black absorber plate
- Copper pipes (risers)
- Insulation
How they work:
- Sunlight passes through the glass.
- Absorber plate heats up.
- Heat transfers into the copper pipes.
- Water or heat-transfer fluid warms as it travels through the pipes.
Pros: Affordable, durable.
Cons: Less efficient in cold, cloudy or frosty climates.
Evacuated Tube Collectors (Premium Option)
These look like rows of glass tubes.
Each tube contains:
- An inner absorber
- A vacuum-sealed outer layer to prevent heat loss
How they work:
- Each tube acts like a tiny greenhouse.
- Vacuum insulation prevents heat escaping.
- Heat transfers via a manifold to the tank.
Pros: High efficiency, great in winter.
Cons: More expensive, fragile tubes.
3. The Two Types of Solar Hot Water Designs
A. Thermosiphon Systems (Tank on Roof)
This system relies on natural heat movement — hot water rises, cold water sinks.
Diagram (in words):
Sun →
[Collector heats water]
↓ Hot water rises
[Roof tank stores hot water]
↑ Cold water falls back to collector
How it works:
- Sun heats water in collectors.
- Hot water becomes lighter and rises up into the roof-mounted tank.
- Cold water from the tank sinks into the collector to repeat the cycle.
- No pump required (lower maintenance).
Best for: Warm climates, simple installations.
Limitations: Heavy roof load, uses smaller tank sizes.
B. Split-System (Tank on Ground, Pump on Roof)
This design uses a circulation pump and controller.
Diagram (in words):
Sensors detect temperature:
If collectors hotter than tank → Pump ON
Water circulates through collectors → heats → returns to tank
How it works:
- Sensors constantly compare roof temperature with tank temperature.
- When roof is hotter, the pump circulates water or glycol through collectors.
- Heat transfers into the tank via direct or indirect heat exchange.
- Pump stops when temperatures even out.
Best for:
- Larger homes
- Cold climates (with evacuated tubes)
- Homes that can’t support a roof tank
4. Direct vs Indirect Solar Hot Water Systems
Direct (Open Loop)
Water flows directly through collectors.
Pros: High efficiency
Cons: Not ideal in frost-prone regions
Indirect (Closed Loop with Heat Exchanger)
Collectors heat antifreeze fluid (glycol).
Heat transfers to tank water through a heat exchanger.
Pros: Frost safe, long-lasting
Cons: More expensive
5. Booster Systems (Winter Backup)
Even in sunny Australia, solar alone can’t guarantee hot water year-round.
Most systems include:
Electric Booster (Inside Tank)
- A resistive element heats water when solar gain is low.
- Works like a standard electric tank.
Gas Booster (Instantaneous or Storage)
- Fires automatically when solar water isn’t hot enough.
- More efficient than electric boosters.
Smart Boosting
Timers or controllers allow boosting:
- Only overnight
- Only on cloudy days
- Only when tank temperature drops too low
This reduces running costs significantly.
6. Frost Protection Methods
Depending on climate, systems may use:
- Closed-loop glycol fluid
- Frost valves that release cold expanding water
- Automatic recirculation to prevent freezing
- Evacuated tubes (naturally frost-resistant)
7. How Much Solar Hot Water Systems Contribute in Australia
Real-world contribution depends on location:
| Region | Typical Solar Contribution |
| QLD / NT | 80–90% |
| NSW Coast / SA | 70–85% |
| VIC / Perth / Adelaide Hills | 60–75% |
| Canberra / Tas | 50–65% |
Systems with evacuated tubes + gas boosting perform best in colder climates.
8. Running Costs & Savings
Solar Hot Water Running Costs
Lowest of all hot water technologies — especially with gas boosting or solar PV.
Typical Savings
- 50–80% off hot water energy costs
- Even higher when paired with rooftop solar panels
Rebates
Solar hot water systems qualify for:
- STCs (federal rebate)
- State-based energy efficiency schemes (varies by state)
These can reduce installation cost by $1,000–$2,500 depending on system size.
9. Maintenance & Troubleshooting
Common Maintenance Items
- Check pump operation (split-systems)
- Replace sacrificial anode (every 4–5 years)
- Inspect collectors for leaks
- Check booster element
- Flush heat exchanger (if indirect system)
Signs of Problems
- Booster running constantly
- Poor winter performance
- Lukewarm water despite sun
- Leaking roof fittings
- Controller showing fault codes
Typical Lifespan
- Collectors: 15–20 years
- Storage tank: 10–15 years
- Pumps: 5–10 years
10. Is a Solar Hot Water System Worth It?
Best For:
- Families
- Homes with good north-facing roof space
- Homes with existing solar PV
- Long-term owners
- High hot water usage households
Not Ideal For:
- Shaded roofs
- Very small apartments
- Homes without sunlight access
- Occasional-use properties
FAQ
1. Does solar hot water work in winter?
Yes — but may require boosting. Evacuated tubes work best in cold regions.
2. How hot does solar water get?
Collectors can exceed 100°C, but tank temperatures are controlled to 60–70°C.
3. Can I run solar hot water with solar PV?
Absolutely. PV + a heat pump or PV + electric booster is extremely efficient.
4. Do solar hot water systems need servicing?
Yes — ideally every 2 years, or annually in colder climates.
5. Is a tempering valve required?
Yes — by Australian law, bathroom outlets must be limited to 50°C.