Controlled Load vs Heat Pump on Solar Soak: Which Is Cheaper?

Controlled load (often called off-peak, or in some places a dedicated circuit tariff) is a cheaper electricity rate for appliances the network can switch on during low-demand periods, usually overnight. Electric hot water is the classic controlled load appliance: the tank heats up in the small hours on a discounted rate and holds that hot water for the day.

It has been a standard feature of Australian electricity for decades, and it exists on your bill as a separate line with its own lower rate. Our guide to reading your electricity bill explains where to find it.

Controlled load made a lot of sense in a grid powered mostly by overnight coal generation: night-time power was cheap and plentiful, so heating water then was the economical choice. The question in 2026 is whether overnight is still the best time, now that midday solar has changed the picture.

Solar soaking takes the opposite timing. Instead of heating overnight, you time a heat pump to heat during the middle of the day, when solar generation is at its peak and daytime power is cheap or free.

The daytime power can be free in two ways: your own rooftop solar (electricity you generate and use yourself), or a free-power window such as the federal Solar Sharer Offer, which gives eligible households a free 3-hour window from 1 July 2026. Our solar soak guide covers the how-to in detail.

The mechanics are simple: a timer on the heat pump sets it to run in the chosen window, and the insulated tank stores the hot water for the evening. Because a heat pump only needs an hour or two to reheat, it fits neatly into a daytime window.

The two approaches differ mainly in when they heat and what power they use.

The key insight: controlled load still charges you something for the power (a low rate, but not nothing), while solar soaking on your own panels or a free window can bring the energy cost close to zero. If you have solar, daytime soaking generally beats paying even a discounted overnight rate, because self-consumed solar is worth more to you than the low feed-in rate you would get for exporting it.

It is easy to frame this as a contest, but for a heat pump owner the honest answer is that the two often overlap. The real decision is usually: should I heat my heat pump overnight on controlled load, or during the day on solar?

• If you have rooftop solar: daytime soaking usually wins, because you are using power that would otherwise be exported cheaply.
• If you have access to a free daytime window (such as the Solar Sharer Offer): the daytime window is hard to beat, since the power in it is free.
• If you have neither solar nor a free window: a good controlled load rate can still be the cheapest option, and it remains a perfectly sensible way to run a heat pump.

Some households even keep a controlled load or overnight schedule as a backup for cloudy days, while soaking on solar whenever the sun is out. A flexible timer makes that easy.

The single biggest saving is not the tariff, it is the appliance. A heat pump uses about a third of the energy of an old resistance element to deliver the same hot water, whichever tariff you run it on. Switching to a heat pump does most of the heavy lifting.

After that, timing fine-tunes the result. If you have solar or a free daytime window, soaking during the day usually edges out controlled load. If you do not, a good off-peak rate is still a solid choice. Because the outcome depends on your tariffs, your solar and your usage, it is fair to say solar soaking can typically lower running costs further, rather than promising a fixed figure.

For the underlying numbers on what a heat pump costs to run across different tariffs, see our running costs guide.

Ready to switch? Get free quotes from local heat pump installers and ask about setting it up for solar soaking.