Heat pumps + solar PV: the daytime-charge trick that drops bills to zero
Here's a number that surprises people: a household with rooftop solar PV and a properly scheduled heat pump pays roughly $0–$15 a year for hot water. Not a typo. The trick is timing — and it's one we set up free on every install if we know you have solar.
Why daytime matters
A heat pump uses electricity to compress refrigerant, which then transfers heat into your hot water tank. It runs for about 4–6 hours per day in total to keep a 270L tank warm. If those hours land during peak time (5–9pm) on grid power, you pay 35–45 c/kWh. If they land during solar production (10am–3pm), you pay 0 c/kWh — your panels are making more than the unit pulls.
Average heat pump draw is ~600 W when running. A 6 kW solar array generates ~4–5 kW between 10am and 3pm. The maths is obvious.
How to set it up
iStore
Open the iStore app → Schedule → set a daily window of 10:00 to 15:00. The unit only heats during those hours. If you run low on hot water (rare with a 270L tank), tap "Boost" for a manual top-up.
Reclaim CO₂
Reclaim has a hardware timer on the controller — set the run window 10am–4pm. The CO₂ refrigerant is slightly more efficient on cold mornings, but for solar timing the same logic applies.
Thermann
Thermann's basic controller doesn't have a clock timer built in. The cheap workaround: a Sonoff smart switch on the dedicated heat pump circuit (your electrician needs to add it). $40 in parts. We do this on request as an add-on.
Real Pakenham numbers
A Pakenham family of four, 6.6 kW solar, iStore 270L on a 10am–3pm schedule:
| Annual cost | |
|---|---|
| Old electric storage tank | ~$950 |
| Heat pump on grid time-of-use | ~$240 |
| Heat pump on solar daytime schedule | ~$15 |
| Annual saving vs old tank | $935 |
After a $1,780 Thermann or $2,400 iStore install, the payback is roughly 2 years. After that the savings are pure gravy. Twenty-year lifetime saving: $17,000+.
Common questions
What about cold winter days when solar is weak?
On overcast Pakenham days the heat pump still runs in the scheduled window, just pulling more grid power. Even at full grid rate it costs less than a gas tank — you'll still come out ahead.
Should I export solar instead and pay grid for hot water?
Almost never. Feed-in tariff in Victoria has been hovering at 3–5 c/kWh. Heating water with that same solar saves you the 35–45 c/kWh you'd otherwise pay. Self-consumption wins by 8–10x.
FAQ
Can I do this with a gas instantaneous heater instead?
No — gas instantaneous heaters don't store hot water. The daytime-charge trick only works with heat pumps (or electric storage tanks).
Does it shorten the heat pump's life?
Slight wear from more starts per day, but modern inverter heat pumps are designed for cycling. We've not seen a Reclaim or iStore fail early from daytime scheduling.
Do I need a battery?
No. The daytime-charge trick works without batteries — that's the elegance of it. If you have a battery, even better, but it's not required.