Solar heating
Solar heating here means diverting surplus photovoltaic electricity into a hot water cylinder instead of exporting it, not solar thermal collectors with a glycol loop and a pump station. A power diverter reads the net flow at the connection point through a smart meter or a CT clamp and modulates that surplus into a resistive immersion heater, holding export close to zero. Select by phase count, element rating, tank volume and whether your inverter already has a controllable relay output. Part of our accessories range, alongside heat pumps. Guide
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- Smart meters
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- Batteries for photovoltaics
- Heat pumps
Solar heating by photovoltaic surplus, not by solar thermal collectors
Two different technologies share the name solar heating. A solar thermal collector is a glazed panel or an evacuated tube array carrying a glycol mixture, driven by a pump station and a differential controller into a coil in the cylinder. The devices in this category do something else: they take the photovoltaic surplus that would otherwise be exported and push it into a resistive immersion heater in an ordinary hot water cylinder. There is no collector, no glycol, no second hydraulic circuit.
The control device is usually called a power diverter, surplus diverter or PV heating controller. It sits beside the consumer unit, watches the net flow at the grid connection point, and continuously varies the power delivered to the element, typically in steps of a few tens of watts, so that instantaneous export stays near zero. Because the element is a simple resistance, the conversion is close to 100 % efficient at the point of use, and the cylinder becomes the cheapest thermal store on the property.
FROM THE FIELD
"The mistake I see most often is a diverter wired to measure the inverter output rather than the incomer. Production tells you nothing about surplus. The CT has to sit on the main tails, on the grid side of every load in the house, otherwise the unit heats water while the kettle is pulling power from the grid and the customer pays twice for the same kilowatt hour."
Ladislav Proc · Photovoltaics specialist, ONSA Plus
How to choose a photovoltaic surplus diverter
Four things decide the specification: how much surplus the array actually produces on an average day, how many phases the supply and the element have, how large the cylinder is, and what else competes for that surplus, usually a battery or an EV charger.
Work through it in this order: confirm the metering point, then check the existing immersion heater and its wiring, then match the diverter rating to the element, and only then set the priority logic and the legionella routine.
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How a diverter modulates power instead of switching it
A relay can only turn a 3 kW element fully on or fully off. If the surplus is 900 W, switching on imports 2100 W from the grid, and switching off exports 900 W. A diverter avoids that by varying the delivered power continuously.
- Burst-fire control passes whole mains cycles and skips others, switching only at the zero crossing. Low electrical noise, well suited to purely resistive loads.
- Phase-angle control fires the triac part way through each half cycle. Finer resolution, but it generates more harmonic content and needs proper filtering.
- Regulation speed matters more than headline accuracy. A unit that reacts within a second or two follows a passing cloud and a kettle switching on without exporting a spike.
- Rating headroom: the diverter must be rated for the full element power, not for the average surplus. A 3 kW element needs a 3 kW rated output even if it rarely runs flat out.
Why the meter has to sit at the connection point
The diverter controls to a target of zero export, so it needs a signed measurement of import and export, not of production. That measurement comes from a CT clamp on the main incoming tails or from a Modbus energy meter installed at the same position, upstream of every load and of the inverter connection. Measuring at the inverter, or downstream of a subcircuit, gives a number that has no relation to what the utility meter sees.
- CT orientation decides the sign. A clamp fitted the wrong way round makes the unit heat on import and idle on export.
- Three-phase supplies need three CTs or a three-phase meter. Whether surplus on one phase offsets consumption on another depends on how the utility meter nets the phases in your country.
- Shared metering: if a battery inverter, a wallbox and a diverter all measure separately, they can fight each other. Prefer one meter feeding the system over a data link.
Tip from practice
Before ordering anything, unscrew the immersion heater cover and photograph the nameplate. You need the element rating, the thread size and confirmation that the thermostat is a mechanical capillary type with a manual thermal cut-out. An element with an electronic thermostat board must not be fed from a modulating output.
Element compatibility and reusing the existing immersion heater
A diverter output is only for purely resistive loads. Most standard immersion heaters of 2 to 3 kW with a mechanical thermostat qualify and can be reused as they are, which is what makes the retrofit cheap.
- Suitable: plain resistance elements with a capillary thermostat and a thermal cut-out, on a dedicated circuit with its own RCD protection.
- Not suitable: elements with electronic thermostats or PTC control boards, heat pump water heaters, and any inductive or switched-mode load. Modulated waveforms damage the control electronics.
- Condition check: heavy scale on the element raises its surface temperature and shortens its life. In hard water areas, inspect or replace before commissioning.
- Wiring: keep the original thermostat and cut-out in circuit as the safety limit. The diverter modulates power, it does not replace temperature protection.
Single-phase and three-phase heating elements
Cylinder elements come as a single-phase resistance, typically 2 or 3 kW, or as a three-element assembly wired in star, typically 3 x 2 kW for 6 kW total. The two need different diverters.
- Single-phase diverter with a single-phase element is the simplest and most common retrofit, and covers most household surpluses up to about 3 kW.
- Three-phase diverter modulates each phase separately and can absorb 6 kW or more. It suits larger arrays and commercial cylinders, and it loads the supply symmetrically.
- Mixed setup: a three-element assembly can be rewired so only one element is fed from a single-phase diverter, leaving the others on the conventional contactor for the boost function.
- Phase imbalance: dumping 3 kW onto one phase of a three-phase supply is fine electrically, but check that it does not push that phase over the connection limit.
Sizing the cylinder against the daily surplus
Heating water takes roughly 1.16 Wh per litre per kelvin. Raising 200 litres from 15 °C to 60 °C therefore needs about 10.4 kWh, and a 300 litre cylinder over the same rise absorbs about 15.7 kWh. Compare that against the surplus your array really exports on a clear day, not against its peak output.
- Cylinder too small: it reaches the thermostat setpoint by midday and the rest of the surplus goes back to the grid.
- Cylinder too large: the surplus never reaches a useful temperature, standing losses grow, and the boost heating still has to finish the job.
- Rule of thumb: a household exporting 8 to 12 kWh on a summer day is well matched to a 200 to 300 litre cylinder with good insulation.
- Stratification: an element in the lower third heats the whole volume slowly, an element higher up gives usable hot water sooner from a small surplus.
Priority against a battery, and legionella
Surplus electricity is worth more in a battery than in a cylinder, because stored electricity can later run any load, while heat can only ever be hot water. The usual order is house loads first, then battery charging, then water heating, then export. Many systems divert only above a set state of charge, for example 80 %, so the cylinder takes what the battery no longer needs.
A diverter cannot be relied on to reach a pasteurisation temperature, because on a dull week it may never lift the cylinder above 45 °C, which is the range where Legionella multiplies. Keep a scheduled high-temperature run of at least 60 °C from the conventional heat source, a boiler, a heat pump or the timed immersion circuit, typically once a week, and leave the cylinder thermostat and blending valve in place.
Diverter or the inverter's own relay output
Many hybrid and grid tie inverters have a multifunction dry contact or a controllable output that closes when surplus exceeds a threshold for a set time. It is a cheaper route because no separate diverter is needed, but it switches rather than modulates.
- Smart relay output: on and off with a threshold and hysteresis, driving a contactor. Simple, and adequate where surpluses are large and steady.
- Manufacturer heating controller: a modulating device tied to one inverter ecosystem, controlled over the same data bus, with full monitoring in the inverter portal.
- Independent diverter: brand agnostic, works with any existing array, and keeps working if the inverter is replaced.
Comparison of surplus heating control options
| Option | How it controls the element | Best suited to |
|---|---|---|
| Single-phase diverter | Continuous modulation to zero export, typically up to 3 kW | Household retrofit with an existing 2 to 3 kW immersion heater |
| Three-phase diverter | Modulates each phase separately, 6 kW and above | Large arrays, commercial cylinders, symmetrical loading |
| Inverter smart relay | On or off above a surplus threshold with hysteresis | Budget installs where surplus is large and steady |
| Manufacturer heat controller | Modulation controlled over the inverter data bus | New builds staying within one inverter ecosystem |
| Solar thermal collector | Glycol loop and pump station into a cylinder coil | A different technology, not a photovoltaic surplus device |
Swipe the table to the left
Frequently asked questions about solar heating
Is this the same as solar thermal collectors?
No. Solar thermal collectors circulate a glycol mixture from a glazed or evacuated tube panel into a cylinder coil. The devices here take surplus electricity from an existing photovoltaic array and modulate it into a resistive immersion heater. No collector or hydraulic circuit is involved.
Can I use my existing immersion heater?
Usually yes, if it is a plain resistive element of 2 to 3 kW with a mechanical capillary thermostat and a thermal cut-out. Elements with electronic thermostat boards, PTC control or heat pump water heaters must not be fed from a modulating output.
Why does a diverter need a meter at the connection point?
Because it controls to zero export, which requires a signed measurement of import and export at the incomer. A CT clamp on the main tails or a smart meter in the same position sees what the utility meter sees. Measuring inverter production instead gives no usable signal.
Do I need a three-phase diverter?
Only if the cylinder has a three-element assembly, usually 3 x 2 kW, or the surplus regularly exceeds about 3 kW. A single-phase diverter with one element covers most households. Three-phase units modulate each phase separately and load the supply symmetrically.
How big should the hot water cylinder be?
Match it to the daily surplus. Water needs about 1.16 Wh per litre per kelvin, so 200 litres from 15 °C to 60 °C absorbs roughly 10.4 kWh. A household exporting 8 to 12 kWh on a summer day suits a 200 to 300 litre cylinder.
Should surplus charge the battery or heat water first?
Battery first in most cases, because stored electricity can run any load later, while heat can only be hot water. Typical logic diverts to the cylinder once the battery passes a set state of charge, for example 80 %, or when it is charging at reduced power.
Does surplus heating protect against Legionella?
No, and it should not be relied on. Surplus alone may leave the cylinder in the 20 °C to 45 °C growth range for days. Schedule a periodic run to at least 60 °C from the boiler, heat pump or timed immersion circuit, typically weekly, and keep the thermostat and blending valve.
Can I use the inverter's relay output instead of a diverter?
Yes, where surplus is large and steady. A smart relay closes above a threshold and drives a contactor, so it switches the full element on or off. That imports or exports whenever the surplus does not match the element rating, which a modulating diverter avoids.
Will it work with a heat pump water heater?
Not from a modulated output. A heat pump water heater is a compressor load with its own electronics and must never see a chopped waveform. Control it instead with a clean switched signal, a relay contact or its own smart interface, if the manufacturer supports it.
Does a diverter need its own circuit and protection?
Yes. The immersion circuit should be a dedicated radial sized for the element, with its own overcurrent device and RCD protection, and an isolator at the cylinder. Keep the original thermostat and thermal cut-out in circuit as the temperature limit.
Diverter sizing, element compatibility and hot water safety rules depend on the specific cylinder, the existing wiring and national regulations.
About ONSA Plus
Why installers across Europe buy surplus diverters from us
ONSA Plus is a European B2B distributor of photovoltaics, battery storage, heat pumps and EV charging stations. In our own EU warehouses we keep over 1,500 products from 25+ brands, including Huawei, Solis, SolaX and Dyness.
We are not just a trading company. We design and service energy systems ourselves, so we answer questions on surplus diversion, metering and hot water control from engineering practice, not from a catalogue. You handle a warranty claim directly with us and you buy per job.
