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Hot Water Heat Pumps: Choosing the Right One

Last Updated: 16th Dec 2024 By Finn Peacock, Chartered Electrical Engineer

Hot water heat pumps are the most efficient way to heat your water anywhere in Australia, south of Brisbane. In QLD and the tropics, a solar thermal hot water system is slightly more efficient.

This article will explain how hot water heat pumps work, and how to buy a good one. I’ll break down the costs, highlight the best models in Australia, and explain how you can take advantage of government rebates. Plus, I’ll cover efficiency, cold weather, noise levels, and installation.

Heat Pump Comparison Table

Compare heat pump prices and specifications side-by-side.

Product Name

Product Name	Apricus	Aquatech X6 Aquatech X6 Aquatech X8	Ecogenica R-series split Ecogenica R-series split Ecogenica R-series all-in-one	Emerald All-In-One Emerald All-In-One Emerald Split	Enviroheat	Evoheat	Haier Monoblock	Hydrotherm	iStore	Quantum	Reclaim	Rheem Ambiheat	Rinnai Enviroflo	Sanden	Stiebel Eltron WWK	Thermann Split Thermann Split Thermann Hybrid
Manufacturer Logo
Product Image
Choose model:	APHP-R290-260	X6	R 215R 290	EE-HWS-A1-220EE-HWS-A1-270 EE-HWS-A1-320	200EH1-14250EH1-15	Evo270-1	HP200M1-U1HP250M1-U1	Dynamic X8 Gen 5	PASHW015-270LD-FL02-R290	150-08AC6-290200-08AC6-290270-08AC6-290340-08AC6-290	REHP-CO2-160GLREHP-CO2-250GLREHP-CO2-315GLREHP-CO2-400GL	HDc270	EHPA250VMAEHPA315VMA	SAN-160SAQASAN-250SAQASAN-300SAQASAN-315SAQA	WWK 222WWK 302	160250315400
Approx price (RRP + GST, excl. installation)	$3,500	$2,295	$2,200	$2,800	$1,800	$3,800	$3,900	$2,800	$2790	$3500	$5000	$4,000	$3,400	$6000	$3,000	$5,223
Tank size	260L	225L	215L	320L	200L	270L	250L	260L	270L	270L	315L	270L	250L	300L	222L	315L
Tank dimensions (mm)	1800 x 620	1580 x 620	1815 x 510	2050 x 600	1670 x 629	1992 x 640	1951 x 630	1800 x 620	1950 x 640	1900 x 650	617 x 1765	1825 x 690	1847 x 627	1891 x 678	1553 x 690	1762 x 624
Refrigerant	R290 (Propane)	R290 (propane)	R290 (propane)	R290 (propane)	R134a	R134a	R290	R290 (Propane)	R290 (propane)	R290 (Propane)	R744 (CO2)	R513a	R290 (propane)	R744 (CO2)	R-134a	R744 (CO2)
Warranty	6 years	5 years	7 years tank, 5 years heat pump	5 years	5 years tank, 1 year electronics	6 years tank, 5 years compressor and parts	7 year tank, 5 year parts and labour	6 years	5 years	5 years tank, 2 years electrics	10 years (wi-fi enabled)	7 years tank, 3 years sealed system	7 years tank, 3 years heat pump, 1 year valves	10 years tank, 6 years heat pump	Cylinder 5 years, valves 1 year	6 years heat pump, 10 years tank, 2 years parts
Additional resistive element?	Yes	Yes	No	No	No	Yes	Yes	Yes	Yes	No	No	Yes	Yes	No	No	No
Time to reheat full tank	5.2 hours	3.2 hours	3.2 hours	3.5 hours	5 hours	3 hours	3.5-5 hours	4.09 hours	4.15 hours	3.71 hours	3.08 hours	3.5 hours	4 hours	3.61 hours	5 hours	Not specified
Operating temperature range	-7 - 43°C	-5 to + 43°C	-7 to + 43°C	-7 - +43°C	-7 to 46°C	-7 - +43°C	-7 to 45°C	-5 - 43°C	-7 to 45°C	-10 to +35°C	-10 to 43°C	-5 to +43°C	-1 to + 45°C	-10 to 43°C	-5 to + 42°C	-10 to + 43°C
Coefficient of performance (COP)	4.13	4.15	5	4.9	3.9	4.27	4.49	4.15	4.8	4.3	5	4.5	5	5	3.94	5
Inbuilt timer?	Yes	Yes	No	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes	Yes	Yes	Yes	No	Yes
Typical power draw	0.7kW	0.9kW	0.65kW	0.6kW	0.5kW	0.94kW	0.43kW	0.88kW	0.94kW	0.87kW	0.85kW	0.7kW	0.94kW	0.87kW	0.64kW	0.95kW
Max power draw	2.8kW	1.8kW	1kW	0.6kW	0.8kW	1.5kW	2.25kW	2.82kW	1.5kW	0.87kW	2.5kW	2.4kW	2.4kW	2.3kW	0.64kW	2.5kW
Max decibels	43dB	42dB	50dB	49dB	46dB	48dB	43dB	49dB	46dB	50dB	37dB	48dB	48dB	37dB	45dB	37dB
Country of manufacture	China	China	China	China	China	China	China	China	China	China	Heat pump - Japan. Tank - Australia.	Australia	Australia	Heat pump - Japan. Tank - Australia	Slovakia	Tank - Australia. Compressor - Japan.
STCs (Zone 3)	21	21	23	22	20	21	21	21	21	20	21	21	21	21	20	22
Datasheet Supplied?	Apricus datasheet	Aquatech X6 datasheet	Ecogenica R-series datasheet (split)	Emerald all-in-one datasheet	Enviroheat datasheet	Yes	Haier Monoblock datasheet	Hydrotherm datasheet	iStore heatpump datasheet	Quantum datasheet	Reclaim datasheet	Yes	Rinnai Enviroflo datasheet	Sanden datasheet	Stiebel Eltron WWK datasheet	Thermann split datasheet
Warranty Supplied?	Apricus warranty	Aquatech X6 warranty	Ecogenica R-series warranty	Emerald warranty	Enviroheat warranty	Yes	Haier warranty	Hydrotherm warranty	iStore heatpump warranty	Quantum warranty	Reclaim warranty	Yes	Rinnai warranty	Sanden warranty	Steibel Eltron warranty	Thermann warranty
More information on brand	Here	Here	Here	Here	Here	Here	Here	Here	Here	Here	Here	Here	Here	Here	Here	Here

Print Table Expand

Table of Contents

1. What Is A Heat Pump Hot Water System?
2. Heat Pump Comparison Table
3. Hot Water Heat Pump Cost
4. What are the Best Hot Water Heat Pumps in Australia
5. Heat Pump Hot Water Rebates
6. Hot Water Heat Pump Efficiency
7. What to Look for in a Good Hot Water Heat Pump
8. How Does A Hot Water Heat Pump Work?
9. Heat Pump vs. Conventional Resistive Hot Water
10. Heat Pump vs. Solar Thermal Hot Water
11. Heat Pump Water Heater Installation
12. Plumbing Considerations
13. Heat Pump Hot Water and Water Quality
14. Integrating Heat Pump Hot Water with your Solar Panels and Battery Storage
15. FAQs
16. Conclusion

What Is A Heat Pump Hot Water System?

Hot water heat humps (often abbreviated to ‘heat pumps’ here in Australia) use the same principles as reverse cycle air conditioners to heat domestic hot water. They transfer heat from the outside air into your hot water tank. Because they transfer heat instead of creating heat, they are the most energy-efficient way to heat water with electricity.

Hot Water Heat Pump Cost

An installed heat pump hot water system starts at about $3,000 for a good budget unit, such as iStore. It goes up to around $5,000 for a more efficient, faster heating premium model, such as Reclaim. This assumes a simple installation using existing electrical connections and no structural work to mount the tank.

Beware of cheap junk!
You can get a hot water heat pump for under $2,000, but in my experience, you’ll likely end up with something noisy, inefficient and unreliable. Cheap, nasty units have given heat pumps a bad name with Australian plumbers. But I’m here to tell you that good heat pumps are quiet, efficient and long-lasting.

What Are The Best Hot Water Heat Pumps in Australia

If we look at online reviews for hot water heat pumps in Australia, we can see which heat pumps are the best according to consumers. We excluded any heat pump that did not have at least 15 reviews on productreview.com.au at the time of writing.

The top-rated heat pump is Haier with an average score of 4.9 stars. iStore has also proven itself as a great budget option with 4.8 stars on 119 reviews.

Reclaim heat pumps, with their CO2 refrigerant, high efficiency, and near-silent running, have a good reputation in the industry. I installed one on my home three years ago, and it’s going great. They are pricey, though, at $5000-$6000 installed.

Quantum, Thermann, Sanden and Stiebel Eltron all have relatively poor average scores at time of writing.

Heat Pump Hot Water Rebates

Australia-wide rebates: STCs

Not a lot of people realise that – similar to the national ‘solar rebate’ – there is also a national ‘hot water heat pump rebate’ which can reduce the upfront cost of your heat pump by about $750.

The government gets really pissed when it’s called a rebate and prefers the discount to be called ‘claiming STCs’ (Small-scale Technology Certificates).

All you need to know is that all good heat pumps with a tank under 425 litres can claim STCs to get the discount and the amount varies based on how much energy they will save typical households.  While a warm climate make heat pumps more efficient, it also reduces hot water use, so fewer STCs are received in warmer zones.

These climate zones rank from 1 through 5:

The STC discount is almost always included in the advertised price of the fully installed heat pump.

Hot Water Heat Pump STC Calculator

Hot Water Heat Pump State/Territory Rebates & Incentives

Victoria	All Victorian households and businesses (barring the state’s largest energy users) can get discounts for upgrading their gas or electric hot water systems. Some solar water heaters may also qualify as long as they meet the eligibility criteria. For upgrading your inefficient gas hot water heater to a heat pump, you can get a discount of around $490. If your heat pump is replacing a electric hot water heater, you can even get up to $1,190.
New South Wales	The New South Wales Government provides a discount on the installation of a hot water system. If you’re eligible, incentives could range between:  – $400 and $670 when you replace an electric water heater with an air source heat pump water heater  – $190 and $310 when you replace a gas water heater with an air source heat pump water heater.
South Australia	Under the Retailer Energy Productivity Scheme (REPS), South Australian homeowners who no longer have gas (or are disconnecting their gas on the same day of installing their heat pump) can get a $847 rebate plus GST on their heat pump purchase. If your home is still connected to gas, the rebate is $245 plus GST. If eligible for the priority group, households get an additional $113 plus GST. The City of Adelaide offers its residents 50% off up to $2000 for replacing gas or wood-burning appliances with an electric or solar-powered appliance. This would include a hot water heat pump.
Queensland	Currently no state-level rebates for heat pumps in Queensland.
Western Australia	Currently no state-level rebates for heat pumps in WA.
Northern Territory	Currently no state-level rebates for heat pumps in the NT.
Australian Capital Territory	If you are with ActewAGL and you are upgrading a gas or electric hot water system, you can get $1,250 off the purchase price of your new heat pump as well as a $250 credit for your next electricity bill. Under the Home Energy Support Scheme, the ACT Government also gives certain concession card holders a rebate of 50% (up to $2500) off the purchase and installation of new energy-efficient products including heat pumps. Eligible homeowners can combine this rebate with a zero-interest loan.
Tasmania	Currently no state-level rebates for heat pumps in TAS. However, the Tasmanian Government does have the Energy Saver Loan Scheme. This scheme offers interest-free loans of up to $10,000 on the purchase of energy-efficient upgrades, including heat pumps.

Hot Water Heat Pump Efficiency

Heat pump hot water is typically three times more efficient than regular electric hot water.
Here’s how it does it:

In the picture above, the heat pump is operating at 300% efficiency. If that sounds like magic pudding, it’s because we measure a heat pump’s efficiency relative to a simple resistive hot water heater that creates heat and converts almost 100% of the electrical energy into heat, whereas a heat pump can produce 3 times the heat for the same electrical energy.

Pro-Tip: The efficiency depends on the outside air temperature. When the air is below freezing, the efficiency will move closer to 100%, and when it is warm outside, it will be closer to the optimum 400%. A good hot water heat pump’s efficiency will vary from about 150% when -10ºC outside to closer to 400% when ~25ºC outside.

What to Look for in a Good Hot Water Heat Pump

Efficiency (STCs)

All else being equal, the more efficient the heat pump, the cheaper it will be to run. The best way to measure this is to look at how many STCs you would receive.

As STCs vary per zone, I chose to compare STCs for tank sizes around 250 litres in the most populous Zone 3:

As you can see, the difference between the main brands is minimal.

Speed of heating (Litres per hour)

A heat pump isn’t much use if it takes too long to heat the water. You don’t want to wait ages for more hot water after the family’s morning showers, and if you have solar panels, you want the water heated during peak sunlight hours to use solar instead of the grid.

Good news: We are currently working on creating a more standardised version of this graph that will use a consistent ambient temperature, a consistent water inlet temperate, and a consistent storage temperature.

Capacity (litres)

Most people buy a heat pump with a 270-350 litre hot water tank as that’s a good size for a typical 4-5 person family. If you are a 1-2 person home and not planning on expanding 150 litres should work well. If you’ve got 6 people or more, don’t get anything smaller than 315 litres. The biggest residential heat pumps have a 400 litre tank.

Type (Split vs. All-in-one)

A Split HWHP (e.g Sanden, Reclaim brands) has the cylindrical water tank separate from the compressor/heat exchanger unit. Advantages are:

• Slightly better efficiency,
• Slightly faster heating rate,
• Slightly quieter.

An all-in-one HWHP such as the iStore or Evoheat is a big cylinder with the heat exchanger on top. Its advantages:

• a smaller footprint, 
• less external pipework, 
• easier to install, 
• cheaper than split systems.

Warranty

Often, the tank and compressor/heat exchanger have different warranties. Don’t accept less than 5 years for the compressor/heat exchanger or less than 10 years for the tank. The best warranty I know of is Reclaim, which offers up to 8 years on the compressor unit and up to 15 on the tank.

Noise

Good heat pumps are quiet, ranging from 37 decibels (an overhead fan at low speed) to 50 decibels (a louder than average fridge).  If they operate during the day to take advantage of solar, noise normally isn’t an issue.  Split systems are usually quieter than all-in-ones.

Refrigerant

The refrigerant is the fluid that transfers the heat from the air to your hot water. Three main refrigerants are used by HWHPs sold in Australia: R134A, Propane and CO2, used in cheap, mid-range and expensive heat pumps respectively. This table shows their pros and cons:

Refrigerant	English name	Pros	Cons	Used By
R134A	HFC (hydrofluorocarbon)	Allows for cheaper heat pump due to lower pressures.	Being slowly phased out in Australia due to horrible Global Warming Potential (~1430x worse than CO2).	Thermann all-in-one, iStore, Midea, Rheem, Evoheat, and all really cheap HWHPs
R290	Propane	Low global warming potential	Highly Flammable	Aquatech, EcoGenica, Qantum, Haier, Rinnai
R744	Carbon Dioxide (CO2)	Efficient and small quantities used make it environmentally friendly	Used at higher pressure, so needs better engineered heat pump, thus more expensive.	Reclaim, Sanden, Thermann split systems.

STCs

Heat pumps receive a number of STCs based on how much energy they’ll save a typical Aussie home in a climate zone.  They provide a consistent way to compare different models based on their energy savings. 

Unlike manufacturer claims, which can vary based on conditions such as air temperature, STCs are based on standardized testing and modelling.  They take into account how climate affects heat pump efficiency and hot water consumption.  

So while heat pumps work more efficiently in tropical Townsville than chilly Hobart, because little hot water is used in the tropics, they receive more STCs in Hobart because they will save more energy in total.

This makes STCs a more reliable indicator of how a heat pump will perform in real-world conditions, helping you make better choices when selecting a model. We’ve put the STC value for each heat pump in the comparison table above.

STCs are used by the government to provide a point-of-sale discount on the cost of heat pumps. The more STC’s the bigger the rebate, because of the better the expected energy (and CO2) savings. The STC discount is almost always included in the prices you see advertised.

How Does A Hot Water Heat Pump Work

A domestic electric heat pump hot water system is similar to a reverse cycle air conditioner. But instead of moving heat energy from outside air into your home, it’s moved into a water storage tank.

Here’s a simple explanation of how it works:

1. Inside the heat pump there’s refrigerant: a special fluid that boils at a low temperature.
2. The heat pump compresses refrigerant gas. This makes it hot so it heats water in the storage tank.
3. Because heat is transferred to the water, the refrigerant cools and mostly condenses into liquid.
4. The refrigerant is then sent to the evaporator coil and the pressure is reduced as it enters.  This causes the refrigerant liquid to expand into gas and as it expands it gets colder. 
5. The evaporator coil is piping with a fan blowing outside air around it. Because the refrigerant is colder than the air it absorbs heat from it.
6. The now warmer refrigerant gas is then compressed and becomes hot and the cycle continues.

In this way the heat pump uses a small amount of electrical energy to transfer a much larger amount of heat energy from outside air to stored water.

The components of a heat pump hot water system making all this magic happen are:

a) Evaporator Coil

This is where the heat pump absorbs heat from the outside air. Refrigerant enters the coil at low pressure and low temperature and absorbs heat energy.

b) Compressor

The compressor increases the pressure of refrigerant gas, raising its temperature. This step is crucial for transferring heat to the tank’s water.

c) Condenser Coil

In the condenser coil, the hot refrigerant gas releases heat to the water in the storage tank. As the refrigerant cools it mostly condenses into liquid.

d) Expansion Valve

This component controls the flow of the refrigerant into the evaporator coil. It reduces the pressure of the mostly liquid refrigerant, causing it to expand into gas and cool further, ready to absorb more heat from the outside air.

e) Water Storage Tank

This is where the heated water is stored until it is needed. The tank is insulated to minimise heat loss.

f) Fan

The fan helps move air over the evaporator coil, improving the heat exchange process by increasing its transfer rate.

g) Refrigerant

This is the working fluid that circulates through the system.  It releases heat as it condenses from gas into a liquid and absorbs heat when it vaporizes from a liquid into a gas.

h) Control System

This system includes thermostats and other controls that regulate the heat pump’s operation, ensuring efficient and safe operation.

Heat Pump vs. Conventional Resistive Hot Water

Heat Pumps are increasingly replacing conventional, resistive hot water systems because they are more energy efficient. But there are pros and cons when compared with resistive water heating:

	Heat Pump Hot Water	Resistive Hot Water
Installed Cost	$3000-$6000	$1,000-$1,500
Efficiency	~300% (yearly average)	98%
Power draw	500-1000W	1,800-4,800W
Warranty	5-8 years	10-15 years
Noise	37-55 decibels	Silent
Heat rate	40-80 litres per hour	40-80 litres per hour

Resistive hot water heaters are smaller, cheaper, with a longer warranty and are silent in operation.

Heat pumps use about 3x less energy, averaged over a year, and are easier to run off solar power because they only pull 500-1000W compared to 2400-4800W.

Heat Pump vs. Solar Thermal Hot Water

Solar thermal hot water systems heat water directly using the sun’s heat. If there’s enough sun, no electricity or gas is required. 

Solar thermal systems take cold water in, heat it with solar energy, and send piping hot water out. They cost from $4,000 for a ‘flat-plate’ system to $8,000 for a more efficient ‘evacuated-tube’ system.

Here’s how they compare with a heat pump hot water system.

	Heat Pump Hot Water	Solar Thermal
Installed Cost	$3000-$6000	$4,000-$8,500
Efficiency	Higher efficiency in warmer climates.  (But less hot water is used.)	Better in tropical & subtropical Australia. (Can use less electricity for boosting compared to HWHP)
Power draw	500-1000W	1,800-4,800W when boosting
Warranty	5-8 years	5 years
Noise	37-55 decibels	Silent
Heat rate	40-80 litres per hour	40-80 litres per hour

Heat Pump Water Heater Installation

Your HWHP must be installed to Australian Standards regarding the plumbing and electrics, the main ones being:

• AS/NZS 3500.4:2001 Plumbing and drainage, part 4: Heated water services
• AS/NZS 3500.1 Plumbing and drainage, Part 1: Water services
• AS 3498:2020 Safety and public health requirements for plumbing products — Water heaters and hot-water storage tanks
• AS/NZS 3000:2018 Australian Wiring Rules

They must also be installed according to the manufacturer’s instructions.

Here’s an overview of some requirements, especially the ones that are commonly overlooked.

Temperature Control

According to Australian standards, hot water has to be stored at 60°C or higher to prevent Legionnaires’ disease, but it has to come out of the taps at 50°C or less to prevent scalding.  

So, the simplest way to protect yourself is to make sure your hot water system operates long enough to reach that temperature every day, even if it starts with a tank full of cold water. This is usually 3+ hours, but for systems with a low tank recovery speed,

Tip: A comfortable water temperature for a hot water tap is 37°C to 42°C. 

Other ways to control Legionnaires (AS 3498:2020 Section 7.2)

• 45% of the tank heated to 60°C daily, or 
• 90% of the tank to 60°C once a week for 32 minutes, or 
• the water being delivered hits 70°C for 1 second within the 7 days prior to delivery

Nerd Fact: If the water is stored at 60ºC and delivered at 50ºC or less, how does that work? A thermostatic mixing valve (TMV) in the context of a water heater is a device that blends hot water from the heater with cold water to ensure a consistent and safe output temperature. The primary purpose of a TMV is to prevent scalding by maintaining the water temperature at a safe level, typically between 38°C and 50°C, regardless of fluctuations in the hot or cold water supply.

Plumbing Considerations

Below is a list of plumbing requirements for heat pump hot water installation.  If you have a good installer, they’ll make sure to carry out every step correctly.  If you have an excellent installer, they will go above and beyond to make sure the installation is problem-free and long-lasting. I’ve bolded commonly missed requirements that are easy for the layperson to spot.

• Use copper or stainless steel pipes within 1m of the heater outlet
• Support any valves to prevent movement during operation
• For smaller copper pipes, provide support every 50 cm
• Avoid wrinkles or flats on pipe bends and soldered joints on new connections
• Install thermostatic mixing valves (TMVs) to blend hot and cold water for safe output temperatures
• Properly support mixing valves independent of piping
• Provide proper base support for the unit
• Ensure correct installation of cold water inlet, hot water outlet, heat pump inlet and outlet (for split units) 
• Ensure the drain runs into stormwater (not onto the floor)
• Insulate hot water pipes with UV-stabilised material, including elbows, valves and the first 300mm of drain
• Allow for proper clearances around the unit (typically 25cm)
• Flush pipework before testing
• Protect pipes in areas with freezing temperatures

Pro-tip: One of the most obvious things to see on a HWHP installation is the pipe insulation, specifically gaps in it. To clear up any disagreements, here’s what the Australian Standard (AS/NZ 3500.4) says needs to be insulated:

8.21 Piping associated with storage water heaters

Piping shall be thermally insulated to achieve minimum R-value as given in Table 8.2.1 for the climate regions identified in Appendix K for Australia and Appendix I for New Zealand, as follows:

(a) All pipework associated with storage water heaters shall be insulated for at least the first 500 mm, or where an external heat trap is fitted, to a point 150 mm down the heat trap vertical leg closest to the water heater.
(b) The primary flow and return pipes, including valves, between an auxiliary heater and a storage water heater.
(c) All relief valves fitted directly to a storage water heater.
(d) All vent pipes up to 300 mm above the maximum operating level of the heated water system.
(e) On multiple installations, where the heated water manifold, including valves, extends to a point at least 500 mm past the heated water outlet branch from the last installation.
(f) On a solar water heater installation, the pipework between a solar pre-heater and an in-line supplementary water heater on both hot water inlet and outlet pipes from both heaters.

The insulation installed in accordance with the above shall be installed so as not to impede the operation of valves.

Here is an example of non-compliant insulation:

Heat Pump Hot Water and Water Quality

Sacrificial Anode

Your heat pump needs a sacrificial anode which will corrode instead of the storage tank. The anode material is chosen based on your local water chemistry:

Scaling

When you heat up water, it can cause a buildup of calcium carbonate, a common mineral in water. This buildup, known as scaling, can clog up pipes, valves, and especially the tubes in solar collectors.

In Australia, water quality varies depending on the region. Some areas have hard water, which has high levels of calcium and magnesium. When hard water is heated, it tends to leave behind calcium carbonate deposits, leading to scaling. Scaling is more likely to occur in areas with hard water, and it can be a problem in heat pump water heating systems, among other appliances.

What is Calcium Carbonate? Calcium carbonate is a white, chalky substance found in many rocks and is a common component in tap water. It’s harmless to drink, but when water containing high amounts of it is heated, it can form a solid layer inside pipes and equipment, causing blockages.

Integrating Heat Pump Hot Water with your Solar Panels and Battery Storage

Integrating Solar Panels

If you have solar panels, your solar electricity is the cheapest electricity available. So it makes sense to power your heat pump with solar as much as possible. 

Heat pumps are great with solar power because:

• They typically draw less than 1000W – easy for most modern solar systems to power even in winter.
• They store energy. You can heat the water up in the day, and use it after sunset. 

To optimise your heat pump, you don’t need a fancy control system — just a simple timer. All good heat pumps have a timer built-in.

Set the timer for 11am, and your hot water will be fully heated by early afternoon, powered by solar electricity on all but the most overcast days.

That’s all you have to do.

One note of caution, though…

When I got my 323 litre heat pump, I put it on a timer, as above.

But I kept running out of hot water.

My teenagers were having 30-minute showers at night, leaving no hot water for the morning.

So I disabled the timer, and just set it to reheat whenever required. 

Luckily, I’ve got a home battery, which can easily supply the 2-3 kWh per night required to replenish the tank.

But that’s unusual. Most households won’t chew through all their hot water before morning showers. If they have teenagers, they practice better parenting than me.

My advice: when you first get your heat pump, put it on a timer. If you start running out of hot water, remove the timer, and heat up on solar during the day and a cheap tariff overnight. You can get cheap overnight electricity for under 20c per kWh, so your top-up should be less than 50c per night. Still much cheaper than conventional electric hot water or gas. Alternatively tell your teenagers to get out of the bloody shower.

Integrating a Home Battery With A Hot Water Heat Pump

If you have a home battery, you should still try to power your heat pump directly from your solar panels during the day. It’s always good to keep as much battery energy as possible available for powering the rest of your house through the night. The good news is – if you are hot water hogs, and need a nightly top-up, any good battery can handle the extra 2-3 kWh easily.

FAQs

Are hot water heat pumps worth it?

A good hot water heat pump will reduce your water heating energy bill by about 70%, whether you currently heat with electricity or gas. If moving to an electric heat pump means you can remove your gas meter altogether, you’ll save even more because you’ll no longer pay the standing charge for connection to the gas mains (although you will have to pay for the removal of your gas meter).

A home with two or three children or one teenager might use 170 litres of hot water per day, which would consume around 10 kWh of electricity or 50MJ of gas. If you pay an off-peak rate of 20c per kWh, that’s $2 per day. It’s a similar amount for gas. That’s about $700 per year. Expect your water heating energy bill to be about $500 lower with a hot water heat pump.

A good HWHP will cost about $3000-$5000 installed (not including any local heat pump rebates). So, in this case, your payback is between 6 and 10 years.

If you use half the hot water, you’ll double your payback period to 8 to 20 years.

If you have more people in your household, your payback will be better—much better if that includes teenagers who take 30-minute showers every night.

Pro-tip: If you currently have instant gas you’ll use roughly 20% less gas than if you have a gas hot water with a storage tank.  This is because you won’t have “standby losses” caused by the tank of hot water slowly cooling down.  This will reduce the savings from switching to a heat pump, but it can still be worthwhile — especially if you quit gas entirely. 

Heat pumps that reduce grid or gas energy consumption can provide a good return.  But if you have a conventional resistive hot water system mostly powered by rooftop solar, the payback time may be too long to be worthwhile.

But unless you have a huge solar system, a conventional electric hot water system on a timer that switches it on during the day isn’t likely to be mostly solar-powered. It can still consume plenty of grid electricity.

This is because homes use the most hot water during winter and periods of cold cloudy weather when solar output is at its lowest.  Heating elements in conventional hot water systems can also be large, with 3.6kW and 4.8kW being common sizes.  This makes it hard for typical rooftop solar systems to provide most of their power.  So switching to a heat pump can still provide a reasonable return to solar households, especially in colder parts of the country.

What are the disadvantages of a heat pump hot water system?

Compared to conventional, resistive hot water systems, heat pumps are more expensive, need more space, make a small amount of noise when heating, and have more moving parts.

The main disadvantages compared to instant gas hot water are that once the hot water is gone, it can take up to an hour to get hot water again, and the stored water loses heat while it is stored—which can be 1 to 3 kWh per day.

Will a heat pump work when it’s really cold outside?

Yes.

Even when the air feels cold, it still contains heat energy unless it’s at absolute zero. Absolute zero is the lowest possible temperature, defined as 0 Kelvin or -273.15°C. At this point, all molecular motion stops, and no heat energy remains.

In practical terms, temperatures we consider “cold,” like -10°C or even -30°C, are still far above absolute zero. Therefore, the air still has heat energy that can be extracted. The refrigerant that heat pumps use can absorb heat at these low temperatures.

The refrigerant boils and evaporates at very low temperatures, even when the surrounding air feels cold. This phase change from liquid to gas absorbs heat from the air, which can then be compressed and transferred to the water in the heat pump system. So, as long as the air temperature is above absolute zero, there is heat energy to extract.

In practical terms, a good hot water heat pump will happily work down to -10ºC, albeit at a reduced efficiency of around 150%. A cheap, nasty heat pump will often switch over to resistive heating in cold temperatures.

WTF is COP?

COP stands for Coefficient Of Performance. It’s how energy nerds compare heat pump efficiency.

I don’t like to compare heat pumps by COP, but in your research, you’ll find most people comparing heat pumps by COP. So what does it mean, and why don’t I like it?

The efficiency of a heat pump is measured by its Coefficient of Performance (COP), which represents the ratio of heat output to energy input. At milder temperatures, such as 10°C (50°F), a typical air-source heat pump might have a COP of around 3 or 4. This means it produces three to four times more heat energy than the electrical energy it consumes. However, as temperatures drop to around -5°C (23°F), the COP can fall to around 2 or lower. This reduction continues as temperatures approach and fall below freezing​ .

For example, at -10°C (14°F), many heat pumps can still operate but with significantly reduced efficiency, potentially dropping the COP to around 1.5 or lower. 

So the problem with comparing hot water heat pumps by COP is that you usually don’t know what ambient temperature the quoted COP was measured at. This can give you a bum steer if you live somewhere where it gets cold.

A better way is to compare STCs for the climate zone you live in, which is why I included those in my heat pump comparison table instead.

Conclusion

Here’s a quick summary of the key points to remember when choosing a heat pump:

• Avoid cheap units: They’re often noisy, inefficient, and unreliable. Stick with trusted brands with great reviews, like iStore, Haier or Reclaim.
• Rebates are available: Government rebates (STCs) lower the upfront costs, and some states and territories offer additional rebates.
• Noise matters: Good heat pumps are quiet (37-50 decibels). 
• Warranty: Aim for at least 5 years on the heat pump and 10 years on the tank.
• Installation: Make sure it’s done by a qualified installer following Australian Standards. Things you can check yourself are proper drainage and well-insulated pipes.
• Pair with solar: If you have solar panels, set a timer to heat water during peak solar production hours. If this leaves you with cold water in the morning, configure it to reheat overnight, too, ideally on an off-peak tariff.