Running a heat pump on solar, off-peak and free midday power

A heat pump is one of the easiest loads in a house to move, because the tank stores the heat. Running it at the right time can cut its running cost to very little, and in a cold climate it also runs more efficiently in warm afternoon air. Here is how each option works.

Should a heat pump run on solar?

If you have rooftop solar, yes, where you can. The Clean Energy Regulator suggests that with rooftop solar you consider "installing a timer to switch on the heat pump during the day while your rooftop solar system is generating" (CER). Your Home notes "most heat pumps" include built-in timers for this, and that heating in the warmer afternoon improves efficiency (Your Home).

energy.gov.au suggests the best hours for using your own solar are 10am to 4pm in summer and 11am to 2pm in winter (energy.gov.au). The ACT Government puts a heat pump's daily use at about 4.7 kWh for a 300 litre tank. Your solar inverter app shows how much you export in the middle of the day, which tells you whether there is room for it.

What is the free midday power window?

Sources: energy.gov.au Solar Sharer Offer and Victorian Midday Power Saver. Both need a smart meter and you have to opt in. You don't need solar panels. The Solar Sharer windows don't shift for daylight saving (Energy Made Easy). Prices outside the free window can be higher on these plans: the Victorian Government says electricity prices outside the hours "may be higher on this offer", so compare it with your current plan.

A heat pump is an ideal load for these windows: the ACT figure of about 4.7 kWh a day is well inside the 24 kWh cap, and the Victorian Government names the electric water heater as one of the loads to shift into the window.

Does the free window work if my hot water is on controlled load?

No. energy.gov.au says hot water systems "may not be able to be shifted if they are on a controlled load", and Energy Made Easy says appliances on a controlled load circuit are charged separately "and will not benefit from the free usage period". To use the free window, the heat pump needs to be on the general circuit with its own timer. energy.gov.au notes that converting hot water from controlled load to solar self-consumption "needs an accredited electrician".

Can a heat pump run on controlled load or off-peak?

Some can, some can't. Your Home explains that Controlled Load 1 (Tariff 31 in Queensland) may provide power for 6 to 10 hours a day and Controlled Load 2 (Tariff 33) about 16 to 18 hours, and that "some heat pump hot water systems are not compatible with either type of controlled load, while others are only compatible with Controlled Load 2". The federal review adds that some models need a continuous supply to defrost in cold weather and "may not be able to make use of off-peak tariffs" (Decision RIS).

Controlled load is also changing. Your Home says controlled loads are increasingly switched on during the day to soak up excess solar, and Energex says its economy tariffs have "moved from managing evening peaks towards solar soaking", with supply typically interrupted in the morning peak and resumed in the middle of the day (Energex).

Which option is best for me?

Do I need a bigger tank to run on a timer?

Possibly. Victoria's buyers guide says a larger tank "may be required for households using timers to reduce the risk of hot water running out between reheating times" (VEU buyers guide). If the heat pump only runs for three hours at midday, the tank has to cover the evening showers and the next morning. See the size guide.

Does a timer affect rebates?

In Victoria it does. Heat pumps on Solar Victoria's product list must have "an end-user configurable integrated timer" or one that can connect to a solar PV system (Solar Victoria). The NSW Government also suggests looking for a built-in timer (energy.nsw.gov.au). The brand overview lists each maker's stated timer and solar features.