What is a solar diverter and how does it work?

A solar diverter is a device that monitors the electricity flowing between your solar panels and the grid using a CT (Current Transformer) clamp installed on the main supply cable. When the solar system is generating more electricity than the household is using — and the surplus would otherwise be exported — the diverter automatically activates the immersion heater in the hot water cylinder, sending the surplus power there instead. The diverter uses phase-angle control (proportional power control) to vary the power sent to the immersion heater in real time, matching the available surplus precisely and minimising grid import and export.

Will a solar diverter work with my existing solar panels?

Yes — solar diverters are compatible with virtually any grid-connected solar PV system regardless of inverter brand, age, or type. They do not interfere with the solar inverter or the DNO connection. The diverter reads the CT clamp signal rather than communicating directly with the inverter. The only hardware requirement is an electric immersion heater in a hot water cylinder. If your property has a combi boiler with no cylinder, a diverter cannot be used without first installing a hot water cylinder and immersion heater.

How much can I save with a solar diverter?

Savings depend on the size of your solar system, your household's hot water usage, and the cost of the energy you would otherwise use to heat water. A typical UK household spending £250–£400/year heating water with gas can save £100–£200 of that by heating water with diverted solar electricity instead. Households with electric immersion heaters as their primary water heating source (and therefore paying higher electricity rates for hot water) can save more — £150–£300/year. The diverter pays back its installation cost (typically £300–£600) in 2–4 years.

Does a solar diverter affect my Smart Export Guarantee payments?

Yes — but beneficially. The diverter reduces your export to the grid, which reduces your SEG income slightly. However, the value of the electricity used to heat water (replacing gas or grid electricity) is typically worth more than the SEG export rate (3–15p/kWh). The avoided cost of heating water by gas or electricity (at 24p+/kWh) is substantially higher than the SEG rate you would receive for exporting the same electricity. For this reason, using a diverter to heat water is almost always a better use of surplus solar electricity than exporting it at the current SEG rates.

What is the difference between the iBoost, Eddi, and Immersun?

All three devices perform the same fundamental function — diverting surplus solar electricity to an immersion heater. The key differences are: iBoost (Marlec) — established market leader, simple push-button interface, wireless sender unit reads export clamp and transmits to the main unit, optional iBoost Buddy display shows real-time status. Eddi (myenergi) — the most feature-rich option, integrates with myenergi's Zappi EV charger and Harvi CT clamp, supports two separate heating loads (e.g., immersion and underfloor heating), excellent monitoring app. Immersun — similar functionality to iBoost, good value, slightly simpler installation in some configurations. For installations where the customer already has a Zappi EV charger, the Eddi is the natural choice for ecosystem compatibility.

Does a solar diverter need to be installed by a qualified electrician?

Yes. Solar diverter installation falls under Part P of the Building Regulations (electrical safety in dwellings) and must either be notified to the local authority or carried out by a registered competent person who can self-certify under a scheme such as NICEIC, NAPIT, or ELECSA. The work involves connecting to the consumer unit (CT clamp on the main tails) and wiring the diverter output to the immersion heater. An Electrical Installation Certificate (EIC) or Minor Works Certificate should be issued on completion, and an Electrical Installation Condition Report (EICR) of the existing installation may be required.

Can I use a solar diverter with a heat pump?

It depends on the heat pump configuration. Air source heat pumps with an integrated hot water cylinder (such as the Samsung EHS or Vaillant arotherm) typically heat domestic hot water as part of their normal operation — a diverter cannot directly control a heat pump compressor. However, some heat pump systems include an electric immersion heater as a backup or boost element, and the diverter can be configured to control this immersion heater. Additionally, myenergi's Eddi can send control signals to some heat pumps via relay or Modbus to request additional hot water heating when surplus solar is available. Seek installer advice specific to your heat pump model.

SOLAR DIVERTER GUIDE

Solar PV Diverter Installation UK: Immersion Diverter Guide

Everything homeowners and electricians need to know about solar diverters in the UK — how they work, popular brands (iBoost, Eddi, Immersun), installation requirements, costs, savings of £100–£250/year, and wiring considerations.

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11 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key takeaways

01A solar diverter (also called an immersion diverter or solar PV diverter) monitors the property's export to the grid and automatically redirects surplus solar electricity to heat water via an immersion heater, rather than exporting at a low SEG rate.
02Popular UK diverter brands include the iBoost (Marlec Engineering), Eddi (myenergi), and Immersun. Each monitors CT clamps and varies the power sent to the immersion heater in real time.
03A solar diverter requires an existing electric immersion heater in a hot water cylinder. Properties with combi boilers (no cylinder) cannot use a diverter. Properties with heat pumps can integrate a diverter but need careful configuration.
04Unit costs range from £200–£400 for the diverter device, plus installation labour of £100–£200. Total installed cost is typically £300–£600 — significantly cheaper than battery storage.
05A diverter can save a typical UK household £100–£250/year on hot water heating bills by using otherwise-exported solar electricity instead of gas or grid electricity for water heating.
06Installation falls under Part P of the building regulations and must be notified via a competent person scheme. The work involves connecting the diverter CT clamp to the consumer unit tails and wiring the output to the immersion heater.

01 · Solar Diverter Guide

What is a Solar Diverter?

A solar diverter — also known as a solar PV diverter, immersion diverter, or solar water heater controller — is a device that maximises the value of a solar PV system by using surplus generation to heat domestic hot water, rather than allowing that electricity to be exported to the grid at a low Smart Export Guarantee rate.

How it monitors surplus generation: A CT (Current Transformer) clamp is installed on the main supply cable at the consumer unit. The diverter continuously reads the current direction and magnitude — when the reading indicates net export (solar generating more than the household is consuming), the diverter activates.
Proportional power control: Unlike a simple on/off switch, a solar diverter uses phase-angle control to vary the power delivered to the immersion heater in real time — matching the available surplus precisely. This means that if 600 W is available, the diverter sends 600 W to the immersion heater, leaving the household's import/export at near-zero.
Hot water cylinder requirement: A solar diverter requires a vented or unvented hot water cylinder with an electric immersion heater (typically a 3 kW element). Properties with combi boilers (which heat water on demand with no storage cylinder) cannot use a diverter without first installing a cylinder.

During a typical UK summer day, a 4 kWp solar system with a two-person household will generate surplus electricity for 4–6 hours. A solar diverter can heat a full 200-litre hot water cylinder using this surplus — providing free hot water for the day with no gas or grid electricity required.

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02 · Solar Diverter Guide

Popular Solar Diverter Brands in the UK

Three brands dominate the UK solar diverter market, each with a strong track record and nationwide installer support:

iBoost (Marlec Engineering) — £230–£290: The original UK market leader. Wireless design with a separate sender unit that clips onto the export cable, communicating with the main iBoost unit positioned near the immersion heater (up to 30 m range). Simple LED display shows status. The iBoost Buddy (£50 extra) provides a wall-mounted display showing live data including surplus diverted, solar generation, and energy saved. Sold through electrical wholesalers nationwide.
Eddi (myenergi) — £290–£350: The most feature-rich diverter, from the Lincolnshire-based manufacturer of the Zappi EV charger and Libbi battery. Full app monitoring (myenergi app), support for two separate heating outputs (e.g., immersion + underfloor heating), integration with Harvi CT clamp for wireless monitoring, and compatibility with the rest of the myenergi ecosystem. The natural choice for customers who already have or are considering a Zappi EV charger.
Immersun (A Heat Ltd) — £200–£260: A cost-effective alternative to the iBoost, popular with budget-conscious customers. Similar functionality including proportional power control, boost button, and real-time monitoring. Wired CT clamp configuration rather than wireless, which some installers prefer for reliability in properties with signal interference.

All three brands are available through electrical wholesalers. The iBoost and Eddi are the most widely sold in the UK market. For commercial or industrial hot water diversion, higher-power variants are available from several manufacturers.

03 · Solar Diverter Guide

Installation Requirements

Solar diverter installation is an electrical installation task that must comply with BS 7671 and Part P of the Building Regulations. The installation process varies slightly between brands but follows a common pattern:

CT clamp installation: The CT clamp must be installed on the live (line) conductor of the main supply cable, positioned between the meter and the consumer unit. This is typically done by the electrician at the consumer unit. For wireless diverters (iBoost, Eddi with Harvi), the CT clamp sender unit clips onto the cable without breaking the circuit. For wired diverters (Immersun), a cable run from the CT clamp to the diverter unit is required.
Diverter unit location: The main diverter unit is installed near the hot water cylinder (typically in the airing cupboard). The output terminals connect to the immersion heater element — usually via a flex outlet plate or direct to the existing immersion heater wiring.
Power supply: The diverter unit requires a mains power supply (typically a switched fused connection unit, 3A). The output to the immersion heater is rated for the full heater load (typically 3 kW / 13A). The existing immersion heater wiring and thermostat remain in place as a backup.
Part P notification: Installation must be notified under Part P via a registered competent person scheme. An Electrical Installation Certificate or Minor Works Certificate must be issued on completion. The customer should receive a copy along with the diverter's user manual.

04 · Solar Diverter Guide

Solar Diverter Costs and Savings

Solar diverters represent one of the best value add-ons available for existing solar PV owners, with a straightforward payback period and no ongoing maintenance costs.

Unit cost: £200–£400 depending on brand and specification. iBoost: approximately £240. Eddi: approximately £310. Immersun: approximately £210. Prices vary by supplier and may fluctuate with energy market conditions.
Installation labour: £100–£200 for a standard domestic installation (approximately 1.5–3 hours for an experienced electrician). More complex installations (long cable runs, airing cupboard access difficulties) may cost more.
Total installed cost: £300–£600 for a typical domestic installation, including unit, labour, and any minor materials (cable, FCU, fixings).
Annual savings: £100–£250/year for a typical UK household, depending on solar system size, hot water usage, and whether gas or electricity is the current water heating fuel. The saving represents the cost of the water heating energy displaced by diverted solar electricity.
Payback period: Typically 2–4 years. The diverter has no moving parts, requires no maintenance, and should last 10+ years — making the long-term return on investment very attractive.

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05 · Solar Diverter Guide

Wiring Considerations

The wiring arrangements for a solar diverter installation require careful planning to ensure compliance with BS 7671 and safe, reliable operation. Key wiring considerations:

Immersion heater circuit: The existing immersion heater circuit (typically 2.5 mm² T&E on a 16A or 20A MCB) should be assessed before installation. Old rubber or cloth-insulated wiring must be replaced — not connected to a modern diverter. The circuit should be on an RCD-protected way in the consumer unit.
Diverter output cable: Most diverters accept input from the existing immersion heater circuit and output directly to the heater element terminals. The diverter's output must be rated for the full heater load (3 kW / 13A). Follow the manufacturer's cable specification exactly.
Thermostat retention: The immersion heater's thermal cutout and thermostat must remain in the circuit and be operational. The diverter does not bypass the thermostat — it controls power input to the element while the thermostat remains the safety mechanism. Do not remove or bypass the immersion heater's thermal cutout.
Earthing and bonding: Ensure supplementary bonding of the hot water cylinder is maintained (or installed if absent). Check for any existing bonding that may be disturbed during access to the airing cupboard.

06 · Solar Diverter Guide

Solar Diverter vs Battery Storage: Which is Better?

Both solar diverters and battery storage maximise the value of surplus solar generation, but they serve different needs and have very different price points. Understanding the comparison helps customers make the right choice.

Solar diverter advantages: Much lower cost (£300–£600 vs £3,000–£10,000+ for battery storage); no degradation over time; no maintenance required; simple installation; fast payback (2–4 years). Best for customers with an existing hot water cylinder who primarily want to offset water heating costs.
Battery storage advantages: Uses stored solar electricity for any appliance — not just water heating; stores energy for evening and nighttime use; enables time-of-use tariff optimisation; provides backup power capability (with certain systems); higher total energy value captured from the solar system. Best for customers who want to maximise self-consumption across all household loads.
Best combined approach: A solar diverter and battery storage are complementary rather than competing. The battery handles evening demand; the diverter handles any surplus that the battery cannot absorb. For systems with a large solar array, both components together can achieve very high self-consumption rates.

For most UK solar PV owners who already have a hot water cylinder, a solar diverter is the logical first step — install it within a year of the solar panels, enjoy the immediate savings on water heating, and consider adding battery storage later if electricity bills remain high.

07 · Solar Diverter Guide

For Electricians: Solar Diverter Installations

Solar diverter installations are a quick, profitable add-on for electricians already working on solar PV or general domestic electrical work. A typical installation takes 2–3 hours, requires minimal materials, and generates a healthy margin at the current market rate. With over 1.3 million solar PV installations in the UK, many of which do not yet have a diverter fitted, the market opportunity is substantial.

Key upselling opportunity: when carrying out an annual inspection or EICR on a property with solar panels and a hot water cylinder but no diverter, always mention the diverter option. The payback period and simple explanation ("it heats your hot water for free using electricity you'd otherwise sell for 3–5p per unit") make it an easy sell to most solar PV owners.

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