A heat pump water heater uses electricity to move heat from the air into the tank rather than to create it, which is why it uses far less energy than an element. This guide explains the refrigerant cycle, COP, cold-weather behaviour and noise. A heat pump water heater does not turn electricity directly into heat the way an element does. Instead it uses electricity to run a compressor and fan that collect heat already present in the surrounding air and move it into the water. Because moving heat takes much less energy than creating it, a heat pump commonly delivers around three to four units of heat for each unit of electricity in mild conditions. That ratio, the coefficient of performance, is the key to understanding both its strengths and its limits.
The refrigerant cycle in four stages
A heat pump uses the same physics as a refrigerator or a reverse-cycle air conditioner. A refrigerant circulates in a sealed loop, changing between liquid and gas, and each change of state carries heat from one place to another.
- Evaporator. A fan pulls outside air across a finned coil. The refrigerant inside is colder than the air, even on a cool day, so it absorbs heat and boils into a low-pressure gas.
- Compressor. The compressor squeezes that gas to a much higher pressure, which raises its temperature well above that of the water in the tank. This is the main step that uses electricity.
- Condenser. The hot gas passes through a heat exchanger wrapped around or inside the tank. It gives up its heat to the water and condenses back into a liquid.
- Expansion valve. The liquid passes through a restriction that drops its pressure sharply, cooling it below the outside air temperature, ready to absorb heat again.
Different refrigerants suit different designs. Some units use propane (R290), which has a low global warming potential, while others use carbon dioxide (R744), which can reach high water temperatures and tends to cope well with cold air. Either way, the energy delivered to the water is the heat collected from the air plus the electrical energy used by the compressor.
Coefficient of performance explained
The coefficient of performance (COP) is the heat delivered divided by the electrical energy used. An element has a COP of 1: one kWh in, one kWh of heat out. A heat pump with a COP of 3.5 delivers 3.5 kWh of heat for each kWh of electricity.
A worked example shows why this matters. Heating 300 litres of water by 45°C takes roughly 15.7 kWh of heat, because water needs about 1.16 watt-hours per litre for each degree. The electricity required depends on the COP:
| Heating method | Illustrative COP | Electricity for 15.7 kWh of heat |
|---|---|---|
| Resistive element | 1.0 | About 15.7 kWh |
| Heat pump, cold morning | 2.5 | About 6.3 kWh |
| Heat pump, mild day | 3.5 | About 4.5 kWh |
| Heat pump, warm afternoon | 4.0 | About 3.9 kWh |
These figures are illustrative; real COP depends on the model, the air temperature, the water temperature and the tank's heat losses. Two physical rules hold for every unit. COP falls as the air gets colder, because there is less heat to collect and the compressor must work harder. COP also falls as the target water temperature rises, because the temperature gap the compressor must bridge grows larger.
Cold weather and defrosting
Cold air is the main challenge. When the evaporator coil runs below freezing, moisture from the air freezes on its fins and blocks airflow, which can happen even when the air itself is a few degrees above zero. The unit detects this and runs a defrost cycle, usually by briefly sending hot refrigerant back through the coil. Defrosting uses energy without heating the water, so recovery slows and effective COP drops on frosty mornings.
This is most relevant in the colder parts of NSW, such as the Southern Highlands, the Blue Mountains and the Central Tablelands. Practical responses include choosing a model rated for low air temperatures, scheduling heating for the warmest part of the day rather than overnight, and siting the unit where it gets winter sun and shelter from cold wind. Many units also include a backup element for very cold spells.
Noise and placement
A heat pump makes a sound similar in character to the outdoor unit of a split-system air conditioner: a fan whoosh and a compressor hum. Manufacturers publish a sound rating in decibels, and a difference of a few decibels is noticeable. Placement matters as much as the rating:
- Keep the unit away from bedroom windows, including a neighbour's.
- Avoid tight corners and narrow side passages, where hard surfaces reflect and amplify sound.
- Mount it on a solid, level base to limit vibration.
- Respect the manufacturer's airflow clearances, which also protects efficiency.
NSW noise regulations restrict heat pump water heaters from being heard inside a neighbour's home during night-time hours, which is another good reason to run them during the day.
Running a heat pump on solar
Heat pumps and rooftop solar pair well for two reasons. First, the electrical input is low, typically in the order of 1 kW while running, so a modest daytime surplus can cover most of it. Second, air is warmest in the middle of the day, which lifts COP. Scheduling the unit to heat between late morning and mid-afternoon therefore gains twice: cheaper energy and more heat per kWh.
A compressor should not be fed rapidly varying power, so a heat pump is controlled by a timer or a smart signal that starts a full heating cycle rather than by a solar diverter. On a time-of-use plan, keeping it out of the evening peak adds further value, as described in understanding time-of-use tariffs. The low value of exported solar, covered in understanding feed-in tariffs, is what makes using that midday surplus worthwhile.
Installation involves plumbing, electrical and, when replacing gas, gas disconnection work, each of which must be carried out by an appropriately licensed tradesperson.
Next steps
Heat-Pump Hot Water is listed in the Blue Energy Solar market from $3,490 supplied and installed, and a Gas to Electric Hot-Water Conversion from $2,990 for homes moving away from gas. Prices are indicative and confirmed after a site assessment. To plan hot water around your solar output and tariff, request a free assessment from Blue Energy Solar.
Frequently asked questions
How long does a heat pump take to reheat a tank?
Heat pumps recover more slowly than a large element because their heat output is lower. A full reheat commonly takes several hours, depending on tank size, how much hot water was used, the air temperature and the model's recovery rating. Households with heavy morning use should size the tank generously and schedule heating so the tank is full before the busiest period.
Is a heat pump water heater the same technology as an air conditioner?
The underlying cycle is the same. A reverse-cycle air conditioner moves heat between indoor and outdoor air, while a heat pump water heater moves heat from outdoor air into stored water. Water heaters come as integrated units, with the compressor mounted on the tank, or as split units with a separate outdoor module. The choice affects noise, placement and installation more than efficiency.
What maintenance does a heat pump water heater need?
Keep the evaporator fins and any air filter clear of leaves and dust, and make sure the condensate drain flows freely, because a heat pump produces water as it cools the air. The tank's relief valve and, for many tanks, the sacrificial anode also need periodic attention from a licensed plumber. Any work on the sealed refrigerant circuit must be done by an appropriately licensed technician.
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