Heat Pump Hot Water with Solar: Best Time to Run It
A heat pump can be a very effective way to use daytime rooftop solar because hot water acts as stored thermal energy. The best schedule depends on solar production, household demand, tank size, recovery time, battery strategy and the controls available on the exact heat pump.
Use your solar setup and household pattern to choose a sensible starting heating window.
Why Daytime Heating Can Make Sense
A heat pump uses electricity to move heat into a storage tank. If that electricity is drawn while rooftop solar is producing more power than the rest of the home is using, some or all of the heat-pump demand can be supplied by solar that might otherwise be exported.
The tank then stores that energy as hot water for later showers and household use. In that sense, a hot water tank can act as a simple form of thermal storage — but only if the heating schedule still gives the household enough usable hot water when it is needed.
A narrow daytime-only schedule can work well when the tank and recovery are large enough. If the household regularly empties the tank before the next heating window, the schedule needs to be widened or the system sizing reconsidered.
When Should a Heat Pump Use Solar?
For many solar homes, the most useful starting point is the middle of the day rather than the very start or end of the solar-production window.
Early heating can compete with breakfast loads before rooftop output reaches its stronger daytime period.
Solar output is commonly stronger and the home may have more surplus available to divert into hot water.
Finishing recovery later in the solar window can leave a fully heated tank ready for evening showers.
Unless a specific tariff or battery strategy justifies it, overnight heating generally cannot use direct rooftop-solar production.
Why Hot Water Can Be a Good Solar Load
Unlike many household loads, hot water can often be shifted in time because the tank stores the heat after the compressor stops.
Daytime electricity can be converted into stored hot water and then used during evening or morning demand.
A heat pump can deliver several units of heat for each unit of compressor electricity, so its electrical draw is often much lower than the thermal energy being stored.
Where the model provides timers or scheduling, hot-water heating can often be moved away from expensive or low-solar periods.
Typical Heat Pump Solar Strategies
These are starting points only. The best window depends on actual solar generation, recovery time and household use.
| Household Setup | Starting Strategy | Why It Can Work | Main Risk |
|---|---|---|---|
| Solar, no battery, evening showers | Late morning to mid-afternoon | Targets stronger solar production and finishes heating before evening demand. | A short window may not allow full recovery after unusually heavy use. |
| Solar, no battery, morning showers | Midday heating with enough tank reserve overnight | Stores daytime solar energy for the following morning. | An undersized tank can run short before the next solar window. |
| Solar + battery | Prioritise direct solar before discharging stored battery energy | Preserves battery capacity for loads that cannot be shifted into the day. | Battery and energy-management settings can change the preferred strategy. |
| Little daytime export | Use a wider or more flexible heating window | There may not be enough spare solar to justify forcing all heating into a narrow midday period. | Over-restricting the schedule can hurt hot-water availability without much solar benefit. |
Should the Heat Pump Use Solar Before the Battery?
Many households prefer to run shiftable loads directly from solar and preserve stored battery energy for the evening, but the best strategy depends on tariffs and the home's energy-management system.
If daytime generation would otherwise be exported, heating the tank can increase self-consumption and store useful energy as hot water.
Where practical, a daytime heat-pump schedule can reduce the need to discharge the battery into a load that could have been shifted into the solar window.
Feed-in tariffs, time-of-use rates and battery settings can alter which option delivers the lowest actual cost.
Your Solar Window Has to Be Long Enough
A perfectly timed solar schedule is no use if the heat pump cannot recover the required heat before the window closes.
Enough stored reserve is especially important when heating is restricted to a defined daytime period.
A slower heat pump needs more time to restore the same amount of thermal energy after heavy hot-water use.
Cooler ambient conditions may reduce heating performance, so a schedule that works in summer may need more margin in winter.
Use the HWO recovery estimator before narrowing the heating schedule too aggressively.
Timer, App or External Control?
Different heat pumps offer different ways to control when heating occurs, and the available method can affect how well the system fits a solar strategy.
A programmable timer can be enough when the goal is simply to move the normal heating cycle into a predictable daytime window.
Connected controls can make it easier to change schedules, boost behaviour or operating modes as seasons and household routines change.
Some installations may use external controls or energy-management systems, but compatibility and electrical requirements need to be checked for the exact product.
Does Plug-In vs Hardwired Affect Solar Use?
The connection method itself does not determine heat-pump efficiency, but it can affect how the electrical and control arrangement is implemented.
A plug-in model can suit properties with an appropriate dedicated outlet, while scheduling is usually handled through the product's own controls or an approved control arrangement.
A hardwired system may suit other product designs or control arrangements and requires appropriate electrical work.
For solar optimisation, focus on whether the system can reliably heat at the desired time rather than assuming one connection type is automatically better.
Choose the Heat Pump Around the Solar Strategy
Once the desired heating window is clear, compare tank size, recovery, controls and current models.
Make sure the tank can carry the household between scheduled heating windows.
Understand how the electrical connection differs before selecting the product.
Compare current HWO heat pumps once your sizing and control priorities are clear.
Common Solar Heating Questions
Quick answers to common questions about using rooftop solar with heat pump hot water.
