Solar, battery and hot water
When should you run a heat pump with solar PV?
Look for a sustained surplus window before heat or hot water is needed. Protect comfort, keep the heat pump’s normal safeguards, and plan how much solar remains for the battery.
Short answer
There is no permanent "battery first" or "heat pump first" rule.
Use surplus PV directly for useful heating when demand exists, the heat pump can run efficiently and the building or tank has safe thermal capacity. Charge the battery when stored electricity will be more valuable later or when reserve is too low.
The decision changes by day. A cold cloudy evening, a clear spring afternoon and an outage-risk morning should not produce the same priority.
Worked example · not a measured home
Share a two-hour solar window between hot water and the battery.
Assume hot water is needed at 18:00 and an ordinary supported heating cycle can run between 12:00 and 14:00 without changing the normal temperature targets or hygiene programme. Its estimated electrical demand is 3 kWh. These are example inputs, not instructions for setting a heat pump.
| 12:00–14:00 budget | Illustrative energy | What to check |
|---|---|---|
| Forecast PV production | 8 kWh | The correct local date, roof and hourly production curve. |
| Other household use | 2 kWh | Include cooking, appliances and any EV charging already planned. |
| Available solar surplus | 8 − 2 = 6 kWh | Surplus before allocating it to hot water or storage. |
| Planned heat-pump electricity | 3 kWh | Electrical input for a useful run, not 3 kWh of delivered heat. |
| Remaining battery-charge input | 6 − 3 = 3 kWh | Stored energy will be lower after charging losses. |
The allocation balances energy across the window, but the actual power still matters. The heat pump may modulate or draw more during part of its cycle. Household peaks, inverter limits or passing clouds can cause imports even when the two-hour energy total looks sufficient.
If the revised PV forecast drops from 8 to 5 kWh, only 5 − 2 = 3 kWh remains after other use. In this example that covers the planned hot-water run and leaves nothing for battery charging. Recalculate the evening battery plan rather than assigning the same 3 kWh to both devices.
If hot water becomes necessary earlier, meet that need. If the tank is already at its normal target, do not manufacture heat demand simply to absorb solar. Exporting or charging the battery can be more useful than raising temperatures beyond the planned operating range.
Check tomorrow’s hourly solar forecast, then use your own heat-pump history to estimate the electrical energy needed for a normal cycle.
Choose a useful window, not the highest single solar point.
- Start with the need. Note when rooms must be comfortable and when hot water must be ready. Preserve the manufacturer’s hygiene and safety programme.
- Check sustained surplus. Compare the production forecast with household use over a suitable run window, not just the sunniest hour.
- Use a supported schedule or interface. Keep normal temperature and compressor limits. Do not rapidly switch the mains supply to follow cloud changes.
- Plan the battery separately. Work out the energy still needed for evening demand and protected reserve.
- Check the result. Confirm hot-water availability, actual electricity used and any imports. Adjust timing only within the installer- and manufacturer-supported options.
Shorter winter days make the overlap harder: demand can rise while the PV window shrinks. Our winter solar-output comparison explains why an annual PV total is a poor guide to how much heating it can cover on a particular day.
Control hierarchy
Optimize only after the hard limits are protected.
- 01Health and equipment safety
Manufacturer hygiene logic, anti-scald protection, freeze protection, compressor limits and grid-code controls.
- 02Comfort and hot-water availability
Room temperatures, household routines, water demand and recovery time.
- 03Battery reserve and power limits
Outage readiness, inverter capacity and the risk of simultaneous high loads.
- 04Heat-pump efficiency
Outdoor conditions, flow temperature, short cycling and useful thermal capacity.
- 05Solar, tariffs and export value
The financial timing decision after the physical constraints are satisfied.
Two stores
Electrical storage and thermal storage have different losses.
A battery can return electricity to many loads, but charging and discharging loses energy and adds battery throughput. A building, buffer or hot-water cylinder can hold useful heat, but higher temperatures can reduce heat-pump efficiency and increase standing losses.
In one 12-month Fraunhofer field study, raising thermal setpoints to use more PV was associated with a 4.0% reduction in space-heating seasonal performance and a 5.7% reduction for domestic hot water. Those figures describe one installation, not a universal penalty.
The right comparison is therefore not "free solar versus battery". It is the value of useful heat now versus the future value of stored electricity, with both loss paths included.
Decision matrix
Different conditions create different priorities.
| Conditions | Likely priority | Checks before acting |
|---|---|---|
| Clear, cold day | Pre-heat usefully during sustained PV while preserving evening battery reserve. | Comfort range, heat-pump run time, forecast duration and evening demand. |
| Clear shoulder-season day | Avoid overheating; battery or export may be more useful than extra heat. | Actual heat demand, tank capacity, export value and battery room. |
| Cloudy, expensive evening ahead | Protect battery and meet heating need at the most efficient available time. | Load forecast, reserve, cheap import window and outdoor temperature. |
| Export-limited solar peak | Use a supported sustained heat-pump window if useful thermal capacity exists. | Minimum run time, device interface, temperature limits and expected surplus duration. |
Control interface
Ask the heat pump. Do not remove its power.
SG Ready is one supported route on compatible heat pumps. It uses defined input states that can request or force certain behavior. The exact response still depends on the model and installer settings.
A HEMS should not imitate control by repeatedly cutting mains power. The heat pump needs its own controller active for compressor protection, circulation, freeze protection, timing and fault handling.
Compatibility must be tested for the exact model and function. The HEMS compatibility checklist shows what that evidence should contain.
Commissioning
Verify the whole loop, not only the command.
- Device responseDid the heat pump enter the expected supported state and complete a sensible run?
- Comfort and hygieneWere room, hot-water and manufacturer safety requirements preserved?
- COP or SPFDid higher targets reduce heat-pump efficiency more than expected?
- Battery impactHow did reserve, throughput and later imports change?
- Whole-home costUse actual import, export and tariff intervals rather than solar self-consumption alone.
- FallbackDoes the heat pump return to normal manufacturer control when HEMS data or communication fails?
The conclusion is deliberately conditional: coordinate both stores around the household plan; do not assign one device permanent priority.
FAQ
Coordination questions.
Should surplus solar charge the battery or run the heat pump first?
There is no universal priority. The better action depends on heat demand, expected solar, battery reserve, tariff, export value, heat-pump efficiency and how much thermal storage is safely available.
Is direct solar use by a heat pump always most efficient?
It avoids battery conversion losses, but raising water or room temperature can reduce heat-pump efficiency and increase heat losses. The temperature target and timing still need limits.
Can a HEMS switch a heat pump off at the mains?
That is generally not a sound control strategy. Use a manufacturer-supported input or API and leave the heat pump powered so its own safety, compressor and freeze-protection logic remains active.
Does SG Ready provide full heat-pump control?
No. It provides defined operating states, but exact behavior depends on the heat-pump model and settings. It is not automatically a two-way telemetry and control interface.
Can energy automation change a hot-water hygiene programme?
Safety requirements come first. Automation should not disable or weaken the manufacturer-defined hygiene programme. Any supported timing change must preserve completion and follow manufacturer and installer instructions.
What is the best time to heat water with solar panels?
Look for a sustained PV-surplus window before hot water is needed, after subtracting other household demand. Follow the heat pump’s supported scheduling and temperature limits. A noon start is not automatically best if clouds, an EV, a tariff or hot-water needs change the available window.
Sources
Heat-pump and control references.
- SMA: integrating an SG Ready heat pump
A manufacturer example of forecast-based heat-pump scheduling with PV production windows and device operating properties.
- SMA: SG Ready connection options
Explains supported SG Ready signaling while the heat pump remains continuously supplied with power.
- German Heat Pump Association: SG Ready interface specification
Defines four SG Ready states and clarifies the difference between recommended and forced operating modes.
- Fraunhofer ISE: field data from a PV, battery and heat-pump home
A measured one-home study showing both increased PV use and an efficiency tradeoff when temperature targets were raised.
- MCS Domestic Heat Pumps Best Practice Guide
Installer guidance on flow temperature, cycling, water volume, hot water and system efficiency.
Make it personal
Forecast your own roof, then get tomorrow by email.
Use your location and system size instead of a generic benchmark. The result includes expected output, uncertainty and the strongest solar hours.