A heat pump can turn cheap midday solar into hot water for the evening. How to set the timer for solar hours, size the tank, compare a heat pump with a solar diverter, and what to check before you switch. A heat-pump hot water system is one of the most effective ways to use solar power you would otherwise export for a few cents. Set its timer to run through the middle of the day, size the tank so one daytime heating cycle covers the evening and next morning, and it can heat your water with around a quarter to a third of the electricity a conventional element would use. Whether a heat pump or a solar diverter is the better choice depends on the age of your existing tank, your household and how much excess solar you have.
Why hot water is the natural home for excess solar
In NSW, feed-in tariffs have fallen and are commonly a few cents per kWh, while electricity bought from the grid in the evening commonly costs 30-45 c/kWh. Every kilowatt-hour you use at home instead of exporting is worth far more than the credit you would receive for sending it to the grid. Hot water is ideal for soaking up that surplus because a tank works like a thermal battery: water heated at noon stays hot for the evening shower and the next morning.
You can see what your exports are really worth with our feed-in tariff comparison.
How a heat pump uses less electricity
A conventional electric storage system heats water directly with a resistive element. A heat pump works like a reverse-cycle air-conditioner in heating mode: a compressor and refrigerant circuit move heat from the surrounding air into the water. As a typical range, a heat pump uses around a quarter to a third of the electricity of an element to deliver the same hot water. Two things make it a particularly good partner for solar:
- Modest power draw. The compressor commonly draws around 1 kW or less while running, so even a smaller solar system on a partly cloudy day can cover much of it. Models with a backup element draw more when that element is boosting.
- Better efficiency in warm air. Heat pumps work more efficiently when the air is warmer, which is exactly when solar output peaks.
Setting the timer for solar hours
Most heat pumps have a built-in timer, or can be switched by a timer or smart relay at the switchboard. Practical starting points for Sydney homes:
- Start around 10 am, once solar production has climbed well above the household's background use.
- Finish around 3 pm, leaving a margin before production falls away.
- Narrow the window in winter to roughly 10:30 am to 2:30 pm, when the useful solar period is shorter.
- Avoid overlaps with other big daytime loads, such as a pool pump, dryer or EV charger, if your solar system is small.
- Leave the temperature controls alone: stored water still needs to reach at least 60 degrees Celsius regularly to control bacteria, and the unit's settings handle this.
Moving a hot water circuit from a controlled-load (off-peak) tariff to general daytime supply is electrical work that must be done by a licensed electrician. It can also change what you pay for that circuit, so check your tariff before switching.
Sizing the tank
A heat pump that runs only in solar hours needs enough stored water to last until the next day's sunshine. As a rough guide:
| Household | Typical tank size | Notes |
|---|---|---|
| 1-2 people | Around 150-200 litres | Compact units suit townhouses and smaller blocks |
| 3-4 people | Around 220-280 litres | Suits most typical family routines |
| 5 or more people, or high use | Around 300 litres or more | Allow extra for long or back-to-back showers |
Location matters too. Heat pumps make a low fan and compressor sound, so they should sit away from bedroom windows and neighbouring living areas; running in the daytime helps here as well. They need clear airflow around the unit, a level base and access for servicing.
Heat pump or solar diverter?
A solar diverter measures what your home is exporting and sends that surplus into the element of an existing electric storage tank, adjusting the power continuously. Diverters are designed for resistive elements, so they are an alternative to a heat pump rather than an add-on to one; a heat pump is switched on by its timer or a relay instead.
| Factor | Heat pump on a solar timer | Solar diverter on an existing electric tank |
|---|---|---|
| Electricity used per tank | Low, typically a quarter to a third of an element | Higher, but drawn from surplus that would have been exported |
| Solar surplus needed | Small; runs on a modest surplus | Larger; works best on bigger systems with regular export |
| Cloudy and winter days | Still efficient, may draw some grid power | Often relies on off-peak or grid boosting |
| Upfront work | Replaces the water heater | Keeps the existing tank and adds a controller |
| Indicative market price | Heat-Pump Hot Water from $3,490 | Smart Solar Diverter from $890 |
A diverter can make sense when your electric tank is relatively new and you export a lot. A heat pump is usually the better long-term choice when the existing tank is near the end of its life, when your solar surplus is smaller, or when you want to remove gas hot water altogether. Our Gas to Electric Hot-Water Conversion covers removing the gas unit, installing the new electric system and its circuit, with the gas disconnection done by a licensed gasfitter. Every option is listed in our energy market, and all prices are indicative and confirmed after a site assessment.
Incentives and approvals
Government incentives may be available when an older water heater is replaced with an eligible heat pump, but availability and value change and depend on the product, the system being replaced and your circumstances. We check what applies to your home when we prepare your quote and show any discount clearly; confirm current values on the official pages before signing. Installation involves plumbing and electrical work, so it must be carried out by licensed trades.
Next steps
If you already have solar and export plenty in the middle of the day, request a free quote and we will look at your hot water system, switchboard and export pattern together during the in-home assessment. If you are still planning solar, our residential solar page explains how we size systems with future electric loads, such as hot water and EV charging, in mind.
Frequently asked questions
Will a heat pump work on cold winter mornings?
Yes. Heat pumps are designed to operate in air temperatures well below what most of Sydney usually experiences, although efficiency falls as the air gets colder and heating takes longer. That is one more reason to run the unit in the middle of the day. In cooler areas such as the Blue Mountains or the Southern Highlands, we check the model's rated operating range and whether boosting is needed.
Can a heat pump run from my home battery at night instead?
It can, but it is usually a poor use of stored energy. A battery is most valuable covering evening lighting, cooking and appliances that cannot be moved to the daytime, whereas a hot water tank can store heat made from midday solar without touching the battery at all. Running the heat pump in solar hours keeps the battery available for the loads that have no other option.
How long does a heat-pump installation take?
A straightforward replacement of an existing electric storage tank in the same location is typically completed within a day. Relocating the unit, converting from gas, adding a new circuit or upgrading the switchboard adds time, and any gas disconnection must be done by a licensed gasfitter. We confirm the scope and the timing in your quote after the site assessment.
Please Login or Sign up to be able to comment