TL;DR:

  • A solar diverter monitors your net export and redirects it to your immersion heater instead of selling it back at low SEG rates
  • Products like the myenergi Eddi and Marlec iBoost+ cost £250–£400 installed and typically pay back in 2–4 years
  • They work with any existing solar system and any standard immersion heater — no boiler or system changes needed

If you have solar panels and a hot water cylinder, you’re almost certainly leaving money on the table. During sunny periods your system generates more electricity than you use, and that surplus gets exported to the grid — earning you somewhere between 4p and 20p per kWh under the Smart Export Guarantee. A solar diverter intercepts that surplus before it leaves your home and uses it to heat your hot water instead. Hot water that would otherwise cost you 20–24p per kWh on an electric immersion tariff, produced for free.

How Solar Diverters Work

A solar diverter uses a current transformer (CT) clamp around your incoming main supply cable to monitor whether your home is importing or exporting electricity in real time. When it detects surplus generation (net export), it activates a triac switch to gradually ramp up the power delivered to your immersion heater element — from zero up to the full 3kW element rating — matching the surplus so that your net export stays near zero.

Unlike a simple on/off switch, a diverter uses phase-angle control or burst firing to modulate power in near-real-time (typically every second or less). This means it never draws more from the grid than you’re already using — it only consumes what would otherwise be exported.

Most diverters also have a “boost” function: you can manually trigger full-power heating from the grid when you need guaranteed hot water regardless of solar generation.

Main Products in 2026

myenergi Eddi

The Eddi is the most popular solar diverter in the UK, made by the same company as the Zappi EV charger. It supports two separate heating loads (e.g. immersion heater + underfloor heating element), has WiFi monitoring via the myenergi app, and integrates with the Zappi for coordinated solar use between EV charging and hot water.

  • Retail price: ~£200–£230 (device only)
  • Installed cost: typically £300–£400 including labour
  • Features: app monitoring, dual output, hub integration (optional), eco/boost modes

Marlec Solar iBoost+

The iBoost+ is a well-regarded alternative, slightly simpler than the Eddi but reliable. It includes a wireless sender unit that clips to your meter tails, meaning the main unit can be installed near your hot water cylinder without needing a cable run to the consumer unit.

  • Retail price: ~£200–£250
  • Installed cost: £280–£380
  • Features: wireless CT sender, optional buddy unit for monitoring, compatible with most immersion heaters

Solic 200 / GivEnergy Heat

GivEnergy’s diverter offering (available through their ecosystem) integrates with their battery and inverter platforms, making it useful if you’re planning a GivEnergy battery installation or already have one. The integration allows smarter decisions: charge the battery first, divert to hot water second, all governed by one app.

Savings Calculation

A standard 180-litre hot water cylinder requires roughly 3–4 kWh to heat from cold. At 24p/kWh (typical off-peak electric), that’s 72–96p per day — around £260–£350 per year if you’re heating entirely with electricity.

A solar diverter won’t replace all of that (you still need grid top-up on low-generation days), but a well-sized solar system in the UK can realistically supply 60–80% of your hot water heating needs across the year, particularly May through September. At 70% coverage that’s £180–£245/year in savings — against an installed cost of £350–£400.

Payback: approximately 18 months to 2.5 years for most UK installations with decent solar exposure. That compares extremely favourably with most home energy investments.

If you’re currently on a solar export tariff paying 4–5p/kWh (common on older fixed-rate SEG deals), the economics are even better: you’re currently earning 4p for electricity you could instead use to displace 24p purchases.

What You Need

  • An immersion heater element in your hot water cylinder. Most UK cylinders have a 3kW immersion element. Unvented cylinders (pressurised) are fine; vented copper cylinders are fine. Heat pump cylinders typically are not suitable (the element is too small and the diverter logic conflicts with heat pump controls).
  • Solar panels already installed. Any grid-tied inverter will work — SolarEdge, Fronius, Enphase, Solis, GivEnergy, etc. The diverter works at the meter level, not the inverter level.
  • An installer (Part P qualified for the electrical connection). Most solar and heat pump installers will fit a diverter in 1–2 hours as an add-on job.

What a Solar Diverter Won’t Do

A diverter is not a substitute for a battery. It only works when you’re generating surplus, and it can only dispatch energy once — to heat water. A battery stores surplus for later use across any appliance. If your main goal is evening self-consumption, a battery is the right tool. If your hot water costs are high and you already have a cylinder, a diverter is typically a much cheaper and faster-payback option to address first.

It also won’t integrate with time-of-use tariffs the way a smart battery system will. Some Eddi users combine it with off-peak boosting (the Eddi can be scheduled to boost from cheap overnight rate electricity), which adds flexibility.

Installation Tips

Ask your installer to fit the CT clamp on the import/export cable (the meter tails), not on the solar output cable. Fitting it on the solar cable measures generation rather than net export — a common mistake that results in the diverter activating even when you’re still drawing from the grid.

Check that your immersion heater element and thermostat are in good condition before installing. A solar diverter cycling a faulty element regularly will shorten its life. Replacing an element (£20–£40 part, £60–£100 labour) before installation is cheap insurance.