Solar PV & Immersion Heaters Guide

A solar PV immersion heater diverter is one of the most cost-effective upgrades you can add to an existing solar panel system. Instead of exporting surplus electricity to the grid at low Smart Export Guarantee rates, a diverter redirects that excess power to heat your hot water tank. The result is free hot water from electricity that would otherwise earn you just a few pence per unit.

In 2026, with electricity prices still elevated and solar panel ownership widespread across the UK, diverters have become a popular way to improve self-consumption and cut gas bills alongside reduced electricity costs. This guide covers how they work, which products are worth considering, and how to calculate whether one makes sense for your home.

Key Takeaways

• A solar diverter monitors your home’s electricity consumption and diverts surplus solar generation to your immersion heater in real time
• Leading products include Marlec iBoost+, myenergi Eddi, and Solic 200, priced between £200 and £600 installed
• A diverter can save £150-£300 per year in gas or electric water heating costs, depending on your system size and hot water demand
• Diverters work with any standard 1kW, 2kW, or 3kW immersion element in a hot water cylinder
• They are most beneficial for households with 3kWp or larger solar systems and high daytime occupancy
• A battery storage system provides more flexibility than a diverter but costs significantly more

How a Solar PV Immersion Diverter Works

Your solar panel system generates electricity throughout the day. At any given moment, some of that generation is consumed by your home’s appliances and lighting. Any surplus, what’s left after meeting your live demand, either exports to the grid or sits unused.

A diverter installs between your consumer unit and your hot water cylinder’s immersion heater. It contains a current sensor (CT clamp) that clips onto your main electricity supply cable and continuously monitors the flow of current. When the sensor detects that your solar panels are producing more than your home is consuming, the diverter sends the surplus to the immersion element.

Crucially, this happens in real time and at a granular level. Rather than switching the immersion on fully at 3kW (which might exceed your surplus and cause an import from the grid), a quality diverter uses proportional control, also called phase-angle control, to send exactly the right amount of power to the element. If you have 1.2kW surplus, the diverter sends 1.2kW to the immersion. As clouds pass and your generation varies, the diverter adjusts continuously.

The Main Solar Immersion Diverter Products

Marlec iBoost+ (Formerly Solar iBoost+)

The Marlec iBoost+ (rebranded from the Solar iBoost+ following Marlec’s acquisition of the product) is one of the longest-established and most widely installed solar diverters in the UK. It uses a wireless sender unit that clips around the main supply cable and communicates wirelessly to the diverter controller, avoiding the need to run additional signal cables.

The iBoost+ includes a tank thermostat input so it stops diverting once the water reaches your target temperature, preventing overheating. A buddy unit can be added to provide an LED display showing current diversion activity and savings. Installation is straightforward for a qualified electrician, and it works with any standard immersion element. Typical installed cost is £250-£350.

myenergi Eddi

The myenergi Eddi is a more sophisticated product with a colour display, Wi-Fi connectivity, and integration with the myenergi ecosystem (including the Zappi EV charger and Libbi battery storage). The Eddi can divert to two outputs simultaneously, for example, a primary immersion and a secondary load like a towel rail element or an underfloor heating system.

The Eddi’s app provides detailed generation and diversion history, and it can be programmed with priority rules, for instance, divert to the immersion from 10am to 4pm, then switch priority to EV charging from 4pm onwards. It also integrates with Agile Octopus and other time-of-use tariffs, allowing the diverter to top up your hot water from cheap overnight grid electricity if solar hasn’t fully heated the tank during the day. Installed cost is typically £350-£500.

Solic 200

The Solic 200 is a compact, no-frills unit that uses a wired CT clamp rather than a wireless sender. This makes it slightly more involved to install in homes where the consumer unit and hot water cylinder are far apart, but it is highly reliable and has no wireless dependency. It uses a proportional control method and includes a basic display showing diversion status. Installed cost is typically £200-£300, making it the most affordable of the three mainstream options.

Other Options

Powerwise and GivEnergy also offer diverter-compatible products. The GivEnergy Giv-EMS energy management system includes a diverter function alongside battery management, making it an option for households upgrading to a full battery storage system where diversion is one feature among several.

How Much Can a Diverter Save?

The annual saving from a solar diverter depends on how much surplus generation your system produces and how much hot water your household uses.

As a rough guide, a 4kWp solar system on a south-facing roof in the UK generates around 3,400kWh per year. A household that self-consumes 50% of that generation directly is exporting 1,700kWh. Without a diverter, that export earns perhaps 12-15p/kWh under the Smart Export Guarantee, £204-£255 per year. With a diverter, some of that export is diverted to heat water instead of being exported.

If the diverter captures 700kWh of that 1,700kWh surplus and uses it to heat water that would otherwise have been heated by gas at 7p/kWh, the saving is £49 in avoided gas. But if those 700kWh were going to be exported at only 12p/kWh anyway (£84 in SEG income), you gain £49 in avoided gas but lose £84 in SEG earnings, a net loss of £35.

This calculation makes clear that the diverter saving depends heavily on your SEG export rate. At 15p/kWh Octopus SEG rates, the economics are tighter. At the basic obligatory SEG rate of just 4-6p/kWh (offered by energy suppliers meeting only the minimum legal requirement), a diverter that substitutes gas heating at 7p/kWh or electricity at 27p/kWh is clearly beneficial.

The strongest case for a diverter is when:

• Your SEG rate is low (under 8p/kWh)
• You heat water with electricity rather than gas (diverter saves 27p/kWh rather than 7p/kWh)
• You have a large solar system (4kWp or more) generating substantial surplus
• Your household uses a lot of hot water (large family, frequent bathers)

In the best case scenario, an all-electric home, low SEG rate, large solar system, high hot water use, a diverter can save £200-£300 per year and pay for itself within one to two years.

Diverter vs Battery Storage

Battery storage and solar diverters address the same problem, what to do with surplus solar generation, but in different ways and at very different costs.

A diverter costing £250-£500 installed converts surplus solar into hot water. It’s simple, robust, and has no moving parts or battery degradation concerns. But hot water is the only “product” it produces, and the cylinder’s capacity limits how much surplus it can absorb. Once the tank reaches temperature (usually within a few hours of good solar generation), the diverter stops and remaining surplus exports anyway.

A battery system costing £4,500-£7,000 installed stores surplus electricity and delivers it back as usable electricity, for any appliance, at any time, day or night. This is far more flexible and valuable. You can run your washing machine, dishwasher, or kettle on stored solar in the evening, avoiding 27p/kWh grid electricity across all of those loads, not just hot water.

For most UK homeowners in 2026, the right sequence is: install a diverter first (low cost, quick payback), then consider a battery once the diverter economics are understood and if your surplus is large enough to warrant the larger investment. Some households install both, the diverter handles hot water during the day, and the battery stores remaining surplus for evening use.

Case Study: Solar Diverter Installation in Shropshire

Background

A family of four near Shrewsbury installed a 4kWp solar panel system in 2023 with a basic SEG tariff paying 5p/kWh for exports. Their hot water cylinder was a 150-litre vented copper cylinder with a 2.7kW immersion element, located in an airing cupboard adjacent to the consumer unit.

Project Overview

The family’s electricity supplier offered only the minimum SEG rate, making exports of limited value. Their gas bill included significant hot water heating costs. A solar installer recommended a myenergi Eddi as the most practical upgrade to improve self-consumption before considering a full battery system.

Implementation

The Eddi was installed in a single half-day visit by an MCS-accredited electrician. The CT clamp was fitted to the main supply tails at the consumer unit, and the output cable ran the two metres to the immersion element terminals. The Eddi was connected to the home Wi-Fi and configured via the myenergi app. The installer set a minimum boost function to ensure the tank reached at least 60°C every 24 hours (a Legionella prevention requirement), using grid electricity overnight at off-peak rates if solar generation had been insufficient.

Results

In the first 12 months, the Eddi diverted 810kWh to the hot water cylinder. The family’s gas consumption for water heating fell by approximately 720kWh of gas (accounting for cylinder heat losses), saving £50 at their gas unit rate. The 810kWh that was diverted would have earned £40.50 at their 5p/kWh SEG rate. Net annual saving: £9.50, plus the convenience of consistently hot water from midday onwards during summer months. The low gas rate meant the diverter saving was modest, but the family’s gas rate subsequently rose, improving future returns.

Expert Insights From Our Solar Panel Installers About Solar Immersion Diverters

One of our senior solar panel installers with over 11 years of experience working on UK residential solar systems shared their view on diverters:

“Diverters are a no-brainer for households on a basic SEG rate or those heating water with electricity. The payback is fast, the installation is simple, and they just work. Where people go wrong is assuming a diverter is always better than a battery, it depends entirely on your situation. If you’re on a premium SEG tariff like Octopus at 15p, a diverter’s economics look quite different than if you’re on 5p. Run the numbers before committing.”

“The Eddi is our preferred specification because of the dual output capability and the Legionella boost programming. With a tank that might not hit temperature naturally in a UK winter, that automated grid top-up is important from a hygiene standpoint. It also means the customer isn’t left with cold water in January and blaming the solar system.”

Frequently Asked Questions

Summing Up

A solar PV immersion diverter is one of the cheapest and most reliable ways to get more value from your solar panel system. By diverting surplus generation to your hot water cylinder rather than exporting it for low SEG rates, you convert electricity you’d otherwise give away cheaply into domestic hot water you’d otherwise pay to produce. The myenergi Eddi, Marlec iBoost+, and Solic 200 each offer reliable performance at different price points and feature levels. Whether a diverter makes financial sense depends on your specific SEG rate, water heating fuel, and system size, but for the right household it can pay for itself in under two years.

Updated April 2026