When it comes to efficiency, hot water heat pumps have long had the definitive edge over alternative technologies anywhere south of Brisbane. Our latest analysis further upgrades this advice: heat pumps today are the most efficient way to heat your hot water, no matter what area of Australia you live.
The Brisbane line once represented the supposed plan Australia had to abandon most of the country during World War Two, but in the energy efficiency world it represents a different line of defense entirely: north of Brisbane being the last area where there was at least some competition to hot water heat pumps.
Heat pumps are more efficient than conventional resistive electric hot water systems as they move heat rather than creating it, consuming way less energy.
However, efficiency comparisons with solar thermal hot water systems, which use roof-mounted flat plates or evacuated tubes to heat water directly using the sun, haven’t been quite so straightforward.
Our advice until now is that heat pumps were definitively the most efficient option south of Brisbane, due to lower ambient temperatures and sun hours. But our revised verdict is that solar thermal doesn’t keep up even in the north, especially if the heat pump is paired with solar panels.
Heat pump hot water is typically three times more efficient than regular electric hot water.
At First Glance, Solar Thermal Still Has The Edge
SolarQuotes’ resident fact-checker Ronald Brakels says that surface-level analysis can give the impression that solar thermal hot water panels still have their noses in front:
“These days typical new panel efficiency is around 22%. Full sunlight is close is roughly 1,000W per square metre. So one square metre of 22% efficient solar panels will produce 220W of electric power under 1,000W of full sunlight. If we use the rule of thumb that a typical heat pump hot water system uses 1/3 as much as electricity as a conventional hot water system, that means it will provide 660W of heat energy to the water, making them 66% efficient,” he says. “Solar thermal hot water panels can transfer up to 80% of the energy in sunlight into heat into water, or 800W, so they beat heat pumps.”
A Deeper Dive Reveals Otherwise
Ronald warns that this isn’t the complete picture, however:
“But this is not a fair comparison! The solar thermal hot water efficiency figure is only for directly converting sunlight energy into water heat energy, while the heat pump figure includes losses such as heat loss from the cylinder overnight, which also apply to solar thermal. When put on an equal footing, the average heat pump comes out ahead.”
Another factor puts solar PV paired with heat pumps well in front, Ronald says:
“But the real killer for solar thermal hot water efficiency occurs when curtailment is considered. This is energy that isn’t used and which effectively goes to waste. In summer, a solar thermal hot water system can produce far more hot water than a household needs. This means much, or most, of the sunlight energy hitting the thermal collectors goes to waste. This can also happen on sunny days outside of summer, depending on hot water use. But with solar PV and a heat pump, if electricity isn’t required for heating water it can instead power the home, charge a battery, or be exported to the grid for others to use. While there are situations in which PV solar can be curtailed, they occur far less frequently than for solar thermal. Once this is factored in it ruins efficiency comparisons for solar thermal hot water, leaving PV solar + heat pumps the clear efficiency kings.”
Another Brisbane line remains in place however: zones 1 and 2 in the map above get a reduced hot water rebate, as hot regions typically have lower hot water use.
Does Solar Thermal Hot Water Retain Any Advantages?
These days, you’re not likely to save money by installing a solar thermal hot water system as opposed to large PV solar system and using a heat pump hot water system or a conventional electric hot water system on a timer.
Even if you don’t have solar PV you’re not likely to save money, as an electrically boosted solar thermal hot water system uses roughly the same amount of electricity as heat pump hot water. But there are a few advantages:
- If you don’t have solar PV with a battery that can provide backup power, solar thermal can keep your showers hot for as long as the grid is down, provided there’s adequate sunshine;
- Solar thermal is silent, while decent heat pumps emit noise from 37-55 decibels when in operation. That’s from quiet as a whisper up to a running microwave oven;
- Provided your roof can support the water tank, solar thermal hot water can save you the ground space a heat pump or conventional hot water system would otherwise require.
While they’re still not likely to save you money, if you’re in or north of Brisbane, they may come closer to paying for themselves because warmer temperatures and clearer winter skies means lower cost solar thermal flat plate collectors rather than pricier evacuated tubes can be used.
But In Most Cases, The Brisbane Line Is No More
These factors notwithstanding, hot water heat pumps generally are a better proposition for most Australians. SolarQuotes founder Finn Peacock has even ripped his perfectly functional solar thermal hot water system off his roof to create space for more solar panels, and added a hot water heat pump instead.
For more, read our guides on solar-powered hot water options and our deep-dive into hot water heat pumps.
About Max Opray
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While heat pumps are efficient they are not in my view the most cost effective All those moving parts just to heat hot water. Hot water diverter using excess solar works great for me and many others. Simple and low cost especially with low price of exports.
In July 2028, I have some hard decisions to make.
To upgrade our solar PV which will require a new roof, or not. The decision is more complicated by our off-grid solar PV boosted, solar thermal, thermosyphon gravity feed hot water system.
This setup means we seldom boost from the grid. This year so far, 20.7 kWh. Some years, zero. In Adelaide.
The local manufacturer of these tanks has retired and good luck finding a plumber with experience in thermosyphon setups. A heat pump would also mean a lot of new plumbing.
What is missing from this comparison is a critical analysis of cost, reliability, long term lifespan and an important alternative option – Solar PV +resistive storage hot water.
I am assessing options for my new home. I already have 9.2kw solar but will double it and add a good size battery this FY taking advantage of the generous battery rebates.
If you have excess solar, this option can make sense if combined with timed switching and reasonable storage size, combined with good siting/insulation to reduce heat loss – particularly in colder climates (though not an issue where I am).
The cost of purchase of the resistive system is a lot lower, the reliability is widely regarded as much better for good brands and maintenance costs are very low. Also, no noise issues or other day to day issues. Lifespan will probably be longer too.
Why is this option not considered particularly in a place Solar Quotes where readers will tend to be more informed? It seems the best option for me.
This article was about hot water system efficiency, and in particular how our advice around solar thermal is changing: resistive isn’t competitive on this measure, which we note at the top. Resistive has other benefits as you say, we’ve compared them with heat pumps here.
I have a gas powered rennai infinity hot water system. How do I calculate if buying a heat pump system is better (cost wise)? I have 15kw of solar pv paired to a 50kw Sigen BESS.
The key is whether doing so allows you to disconnect from gas entirely, saving you the supply charge we have some analysis on average savings per city from quitting gas, but it won’t be specific to your circumstances.
I feel there is another issue: Roof mounted solar thermal systems are far more expensive to install. In the last few years, I replaced a hail damaged roof. My old Solar Edwards Stainless system was flushed, moved & re-installed because it would have been difficult to install a heat pump. I used new//s/h panels because my old panels were u/s.
I am now contemplating building a small PassivHaus (Passive House), that will use a heat pump, but I will hot feed a resistance HWS for the Kitchen/bathroom due to line length. The small HWS will be heated from PV via timeswitch & contactor. In effect I will only be heating the small amount of cold water until the HW comes thru. Probably a 50Lor 80L system.
Agreed solar thermal HWS are now ineffective because of curtailed heating potential. But regular REHWS hooked up to active timers/contactors that work with a home’s solar system are really solar HWS too but with no curtailment issues and practically cost nothing to run if running on solar power. Other advantages of REHWS are: simplicity, ability to customise electric elements to suit solar power availability, can have dual elements for very quick recovery, noise free, no ventilation requirements, can be installed indoors and are inexpensive.
Great article, but I think the comparison is not pragmatic. The heat pump option is more efficient per m2 of sunlight collected (which I think is your point and a good bit of knowledge), but the rooftop HWS comes with its own solar collectors, so sunlight energy collected is greater.
What I mean is, people are typically not choosing between an extra 2kW of panels on their roof (plus a heat pump HWS) versus a rooftop HWS. For most, they already have a solar system installed or designed and the decision is whether to add a rooftop HWS or a heat pump.
Anecdotally, in Brisbane we have a rooftop solar HWS and a smart circuit to both measure the electricity used by the booster, and to only provide electricity in certain circumstances (to help with the curtailment issue you mentioned). Last year we used around 100kWh total on hot water for the year (1.8kW booster for around 50hrs).
Compared to 2-3,000kWh for heat pump, or about 2kW of extra panels based on my 2024 total generation.
Efficiency is just one measure in which heat pumps have overtaken thermal systems: our recommendation is that they also beat them on install costs and running costs. But thermal hot water systems are great and there’s little need to replace an existing one that’s working well (unless you are very keen to maximise rooftop PV).
Hello
I have a 300L stainless solar thermal which was installed in 2009. I have not replaced the element or the temperature controller. It is on a timer set up. In the last twelve months I have use 2251 kwhs of power, and the new rate for off peak hot water is $0.20735 with gst included, so that comes in at about $467.00 for the year to have hot water available 24 hours a day.
I do use hot water for a number of clothes washes each week.
I understand heat pumps have a shorter life than solar thermal, which could make heat pumps more expensive over the life of hot water systems, than solar thermal. Replacing the solar thermal is not going to be cheap, so when it comes to that, I will be considering my life expectancy, since another solar thermal will probably out live me.
My understanding is
– That heat pumps can not use small amounts of solar energy, like a resistive hot water system can using the right sort of diverter. they need the full whack to run.
– Most electrical authorities (if not all) will not allow you to connect a hot water system to both off peak power and your solar, so if you dont generate enough power to heat your water, you are paying full grid price.
– A battery may nullify both of these issues
These factors combined mean you really need to make sure you have excess power available in winter to either directly run, (or to top up the battery), before dropping the hammer on a heat pump if you have solar. Otherwise if you have to buy power in winter, buying off peak hot water power is likely the cheapest power you will be able to buy, but that might mean paying for that off peak power for your heat pump for 9 or 10 months when you dont need it.
So the heat pump might be cheaper to run, working out how to power it might be tricker.