Solar Soak Hot Water: Run Your Heat Pump on Midday Solar

Solar soaking is the practice of deliberately running an energy-hungry appliance in the middle of the day, when solar generation is at its peak, so it uses cheap or free daytime power instead of expensive grid electricity in the evening. Hot water is the classic solar-soak load because a tank stores the heat for later.

The logic has changed with the grid. For decades the standard advice was to heat water overnight on a cheaper off-peak or controlled load tariff. But the middle of the day is now often the cheapest time to use power, thanks to abundant rooftop and grid solar. Shifting hot water heating from overnight to midday is increasingly the smarter move.

There are two ways your midday power can be cheap or free:

• Your own rooftop solar. If you have panels, electricity you generate and use yourself is effectively free, and worth more to you than the low rate you get for exporting it.
• A free or low-cost daytime window. Even without your own panels, some plans offer very cheap or free midday power. The federal Solar Sharer Offer, for example, gives eligible households a free 3-hour window from 1 July 2026.

The simplest and cheapest way to solar soak is a timer. Most heat pump hot water systems have one built in, or a smart control that does the same job.

Instead of letting the system heat whenever the water cools (which can mean expensive evening or overnight top-ups), you set it to do its main heating during peak solar hours, typically late morning to early afternoon. The tank then holds that hot water for the evening.

Why this works so well with a heat pump: a heat pump only needs an hour or two to reheat a tank, and it draws a modest amount of power (often under 1 to 1.5 kW). That means even a small rooftop solar system can cover it, and it fits neatly inside a short cheap-power window.

Getting the timing right:

• Set the heating window to sit inside your sunniest hours, or inside a free-power window if you are on a plan that offers one.
• Make sure the tank is big enough that one daily heat lasts until the next day. Our sizing guide helps here.
• Limit the electric boost so it does not quietly top up on full-price power at night.

A solar diverter (sometimes called a hot water diverter or PV diverter) takes a different approach. Rather than relying on a timer, it watches your rooftop solar in real time and sends any surplus generation, the power you would otherwise export to the grid for a low feed-in rate, into your hot water tank.

The appeal is that it uses solar you have already paid for and would otherwise sell cheaply. On a sunny day the diverter quietly tops up the tank whenever your panels are producing more than the house is using.

A few things worth knowing:

• Diverters were designed around resistance-element tanks. They work by dialling the amount of surplus power fed to a heating element up and down. A conventional electric storage tank is the natural fit. A heat pump does not use power in the same variable way, so whether a diverter suits your specific heat pump depends on the model. An installer can advise.
• With a heat pump, a timer is often enough. Because a heat pump needs so little energy, simply timing it to run during solar hours frequently captures most of the benefit without extra hardware.
• Diverters add upfront cost. They make most sense for larger solar systems with regular midday surplus and a compatible tank.

You can solar soak a plain electric-element hot water tank, and many people do. But a heat pump makes the whole exercise easier and cheaper.

A resistance element converts one unit of electricity into roughly one unit of heat. A heat pump moves heat rather than making it, so it delivers the same hot water for about a third of the electricity. That has two practical effects for solar soaking:

• A smaller solar system can cover it. Because a heat pump draws so little, even a modest rooftop array (or a short free-power window) can supply all the energy the tank needs.
• You stay under any daily free cap easily. Where a free window has a cap, a heat pump's low draw means a full reheat comfortably fits inside it.

In short, an element can be soaked but needs a lot of surplus power to do it. A heat pump sips power, so it turns even a little midday solar into a full tank of hot water.

For most homes with a heat pump, yes, and it usually costs nothing to set up because it is just a timer setting. Shifting your water heating off expensive evening or overnight power and onto cheap or free daytime power is one of the easiest running-cost wins available.

How much you save depends on your situation: the size of your solar system, your tariff, how much hot water you use, and how well the timing is set. Because of that, it is honest to frame solar soaking as something that can typically cut your hot water running costs, rather than a fixed or guaranteed saving.

The bigger investments (a diverter, or upsizing your solar) make sense in narrower cases, mainly larger households or homes with lots of midday surplus. For everyone else, a well-timed heat pump captures most of the benefit.

If you are in a Solar Sharer state, our Solar Sharer hot water savings calculator gives a quick estimate of what timing a heat pump to the free window could save you. It is a guide, not a quote.

Thinking about a heat pump set up for solar soaking? Get free quotes from local installers and ask them to fit a timer or smart control.