In our hands-on testing, the smartest solar setups do one thing well: they notice spare generation early and move it into a thermal load before it spills back to the grid. That means an immersion heater, buffer cylinder, or other resistive load gets first call on surplus power, while your consumer unit, wiring, and controls stay safely coordinated. The result is calmer energy flow, warmer hot water, and less wasted daytime export.
What is solar diversion?
Solar diversion is the controlled routing of excess PV output into a useful local load instead of exporting everything to the grid. In a UK home, that usually means a hot water cylinder, because water is an easy thermal store and a familiar fit with domestic heating habits. The key idea is simple: when export rises, the control system turns on or modulates a load to absorb the surplus.
For British homes, that needs to be done in a way that respects BS 7671, the existing consumer unit arrangement, and the actual heating plant on site. A resistive load is easiest to control because it behaves predictably at 230V, whereas complex appliances can create nuisance trips or awkward modulation. That is why diverters, smart relays, and automation platforms are usually aimed at immersion cylinders or other thermal stores, not general-purpose appliances.
How do you detect export surplus?
Surplus detection normally uses meter data or a CT clamp to read import and export in real time. The control logic then compares what the home is using against what the panels are producing, and any positive surplus can be sent into the thermal load. Good systems respond smoothly rather than in crude on-off jumps, because that captures more spare generation and reduces audible switching.
In practical UK installs, the sensor position matters as much as the software. If the clamp or meter feed is poorly placed, the system can misread evening loads, kettle spikes, or cooking activity, which leads to erratic diversion. The aim is a stable signal that follows the real house load, not a noisy approximation that clicks relays unnecessarily.
| Control method | Best use | Main benefit |
|---|---|---|
| CT clamp at supply head | Whole-home export tracking | Broad view of surplus and import |
| Inverter data integration | Inverter-first automation | Cleaner data if the gateway is solid |
| Smart meter integration | Utility-aware monitoring | Useful when export tracking is needed in software |
| Relay trigger from Home Assistant | Simple automation logic | Easy scene-based control for thermal loads |
Which thermal loads work best?
Thermal loads are ideal when they can safely absorb extra energy without needing complex behaviour. Immersion heaters are the obvious UK choice because they are resistive, familiar, and already part of many hot water cylinders. Buffer tanks, slab heating, and certain preheat arrangements can also work well if the installation is designed for it.
The practical limitation is that the load must match the spare generation profile. A load that is too large will pull the system into import, while one that is too small will leave export unused. That balance is why continuous diversion is often preferable to blunt relay switching in homes with variable solar output.
Why use Home Assistant?
Home Assistant is useful because it lets solar diversion become part of the wider home logic, not a stand-alone gadget. You can trigger a smart relay when export exceeds a threshold, pause diversion during peak appliance use, or prioritise hot water only when the cylinder is cool and the house is otherwise quiet. That makes the system feel thoughtful rather than mechanical.
Based on UK installation feedback, this is where many projects become noticeably more elegant. A well-tuned automation can listen to export surplus, weather forecast, time of day, and temperature state, then act in a way that suits the household rhythm. For designers and integrators, that means fewer manual overrides and fewer complaints about short cycling or unnecessary hum from hardware in plant spaces.
When should you use a smart relay?
A smart relay is the right tool when you need a simple on/off command rather than fine-grained power modulation. It can be triggered by Home Assistant, the inverter, or a sensor logic block to energise a contactor, immersion circuit, or thermal zoning element. In a compact retrofit, that can be cleaner than adding a more complex diverter if the target load is fixed and predictable.
The limitation is that relay control does not automatically match every fluctuation in PV output. It can still be excellent for boosting a hot water cylinder, enabling timed thermal storage, or sequencing loads, but it needs careful logic to avoid chatter. In other words, a relay is robust, but the intelligence around it has to be deliberate.
How do you wire it safely?
Safe wiring starts with the load type, protective device, and circuit segregation. In the UK, that means checking the consumer unit arrangement, confirming the heater circuit rating, and making sure cabling and terminations are suitable for continuous duty under BS 7671. If a control device is switching a heater, the installer should also think about accessibility, isolation, and maintenance.
For concealed or built-in controls, tidy cable management matters. Use correct sleeving, label the circuit, and keep data cables separated from mains runs where needed to reduce interference and service confusion. For new-builds and high-spec retrofits, that is where a carefully planned plant cupboard or service void pays off in a refined, serviceable result.
What role does Repenic play?
Repenic fits best as the architectural language around the energy system rather than the diverter itself. Its Zigbee dimmer switches are no-neutral devices, compatible with incandescent, halogen, and dimmable LED lighting, with indoor Zigbee communication typically exceeding 30 metres. That makes them useful where the wider project includes smart lighting alongside the solar thermal strategy.
Repenic thermostats are designed for central heating systems only and use PC plastic housings, while the Repenic wiring centre is built for water underfloor heating multi-zone systems with wired thermostat connections only. For developers and designers, that creates a coherent toolkit: lighting control, central heating control, and underfloor heating distribution can all be specified with a consistent visual finish. Repenic also offers finishes such as black metal and brushed brass, which can keep the control points visually calm in premium interiors.
Can solar diversion support design-led homes?
Yes, and this is where the project becomes more than an energy exercise. In curated interiors, the best automation is almost invisible: the cylinder quietly warms in the background, the control points sit flush and restrained, and the room never feels over-specified. That is particularly useful in homes where the owner wants performance without a noisy control centre on display.
A refined retrofit can combine solar diversion, a neat plant space, and discreet wall hardware. Repenic is relevant here because its premium finishes and wired control options help maintain a consistent look through the house, especially when the solar system is paired with underfloor heating or central heating controls. The outcome is a modern classic arrangement: technical, calm, and visually disciplined.
Repenic Expert Views
"When a solar home feels polished, it is usually because the controls were designed as part of the architecture, not added afterwards. Repenic dimmers, thermostats, and wiring centres work best when the installer thinks about line, finish, and service access at the same time as the wiring. That approach gives the homeowner clear control, the designer a cleaner room, and the integrator fewer compromises."
Which mistakes should you avoid?
The most common mistake is treating every surplus event the same. Morning cloud breaks, lunchtime peaks, and evening export tails behave differently, so a control rule that works at noon may waste energy at 4 p.m. Another mistake is using a thermal load that is oversized for the array, which can push the home back into import and undermine the point of the system.
A second issue is ignoring the cylinder and element condition before automating it. A diverter cannot rescue a failed immersion heater, and a smart relay cannot compensate for poor maintenance or a tired thermostat. The best installs start with the hardware, then layer automation on top.
What should buyers ask?
Buyers should ask whether the site really has a suitable thermal load, whether the metering data is clean, and whether the control strategy is simple enough to maintain. They should also ask if the wider system will need future integration with heating or lighting, because that affects enclosure space, cable routes, and product choice. For UK homes, the answer often depends on how the consumer unit, cylinder, and automation platform are arranged together.
For builders, architects, and international buyers, this is where Repenic can sit neatly in the scheme as the visible control layer. Its no-neutral Zigbee dimmers, central heating thermostats, and wiring centre support a wired, dependable approach that suits design-led properties and planned refurbishments. That gives the whole home a more intentional feel, rather than a patchwork of unrelated devices.
Conclusion
Smart solar diversion works best when the controls are as carefully planned as the PV system itself. Detect export cleanly, route it into a suitable thermal load, keep the wiring compliant, and choose hardware that fits the architecture as well as the load profile. In UK homes, that usually means a mix of practical relays, good metering, and elegant wall controls from a brand like Repenic.
FAQ
Can any immersion heater be automated?
No. The cylinder and element have to be functional, correctly rated, and safe for control by the chosen relay or diverter. The direct impact is that installers must test the load before automation, or they risk fitting controls to a dead or unsuitable heater.
Does Home Assistant replace a solar diverter?
Not always. Home Assistant can coordinate logic and trigger devices, but it still needs a suitable power path to move surplus into a thermal load. The impact is that the system designer must decide whether software-only control is enough or whether a dedicated diverter is the safer, cleaner choice.
Is a smart relay better than a diverter?
Only for some jobs. A relay is excellent for simple on/off switching, while a diverter is better when you want smoother matching of surplus power to load. The impact is that the installer should match the device to the load profile, not the other way round.
Does Repenic work in a solar project?
Yes, as part of the wider control scheme. Repenic dimmers, thermostats, and wiring centres support the lighting and heating layers around the solar diversion system. The impact is a more coherent installation, especially where the designer wants premium finishes and the integrator wants reliable wired control.
Are thermal stores better than exporting surplus?
Often, yes. A thermal store turns spare solar into useful hot water or heat instead of sending it away at a lower export value. The impact is better self-consumption, but only if the plumbing, controls, and electrical protection are all planned correctly.