What a solar diverter does (and does not do)
A solar diverter is an electronic device that monitors the flow of electricity at your meter. In real time, it measures how much electricity your solar panels are generating and how much your home is consuming. When generation exceeds consumption, the surplus would normally flow out to the grid. The diverter intercepts that surplus and redirects it to your immersion heater, heating the water in your hot water cylinder instead.
The diverter does not store electricity. It does not charge a battery. It does not directly power anything other than the immersion heater. Its sole function is to catch electricity that would otherwise be exported at low rates and use it to heat water that you would otherwise have to heat with gas or paid electricity.
A diverter operates across a wide range of surplus levels. It does not require the full 3kW that an immersion heater typically draws. When surplus is only 500W, it sends 500W to the heater. When surplus is 2.5kW, it sends 2.5kW. This proportional control means no electricity is wasted, and the heater runs as hard as the surplus allows at any given moment. On a sunny summer afternoon, a 3.5kWp system can push a full 2-3kW to the immersion heater for several hours, heating a 200-litre cylinder from cold to the target temperature of around 60 degrees Celsius entirely for free.
In summer, a household with a 3.5kWp solar system and a diverter can heat its hot water entirely from solar for typically five to seven months of the year. Annual water heating cost during this period: zero.
The economics: SEG export vs hot water value
The financial case for a solar diverter rests on a simple comparison. What do you earn by exporting that surplus solar electricity? What do you save by using it to heat water instead?
Exporting 1 kWh earns you the SEG rate. Typical UK SEG rates in 2026 range from 4p/kWh (minimum supplier rates) to 12p/kWh (competitive rates). Octopus Outgoing pays approximately 15-20p/kWh, which is among the best available. Take 15p/kWh as a favourable export rate.
Using 1 kWh to heat water with an electric immersion heater saves you the cost of heating that water another way. If your backup water heating is a gas boiler at 5.5p/kWh gas rate (with a typical boiler efficiency of 90%), the effective cost of heating 1 kWh of water with gas is approximately 6p. If your backup is an electric immersion heater on a standard tariff at 26p/kWh, the cost is 26p per kWh of heat.
When you run the comparison: if you have a gas boiler as backup and a 15p/kWh export rate, exporting earns 15p while diverting saves 6p. In this case, exporting is financially better. If your backup water heating is electric (many properties without gas connections), diverting saves 26p per kWh versus earning 15p by exporting. The diverter wins clearly.
The honest conclusion: for households with gas backup water heating and a good SEG rate of 15p+, a diverter makes a smaller financial difference than for households relying on electric immersion for backup. But the diverter still adds environmental value and simplicity. For households with electric-only water heating, the diverter is an excellent investment regardless of your SEG rate.
Best solar diverters UK 2026
Three products dominate the UK solar diverter market in 2026, each with a distinct price and feature profile.
myenergi Eddi: The premium choice. The Eddi is part of myenergi's ecosystem alongside the Zappi EV charger and the Libbi home battery. It integrates with the myenergi app, gives detailed monitoring of how much solar electricity has been diverted and how much water heating cost has been avoided, and works with Home Assistant for advanced automations. The Eddi can also be set to draw from the cheap Agile overnight window for backup water heating when solar is insufficient. Installed cost: approximately £350-450. Warranty: two years.
Solic 200: The reliable budget option. The Solic 200 has been on the UK market since 2012 and has a strong reputation for reliability and simplicity. There is no app, no monitoring dashboard, and no smart integration. It does one thing: diverts surplus solar to your immersion heater. For households who want a fit-and-forget solution without any connectivity or monitoring requirements, the Solic 200 is an excellent choice. Installed cost: approximately £200-300.
iBoost+: The mid-range option with app control. The iBoost+ offers wireless monitoring via a display unit and a basic app interface. It records how much energy has been diverted and provides a straightforward boost function for manually triggering the immersion heater when needed. A solid choice for households who want some monitoring capability without the full smart home integration of the Eddi. Installed cost: approximately £280-380.
All three products are compatible with most standard single-phase UK solar installations and all standard immersion heaters. The choice between them comes down to your preference for monitoring, smart integration, and budget rather than any fundamental difference in diverting performance.
Installation cost and typical payback period
Solar diverter installation is typically a straightforward job for a qualified electrician. The diverter unit mounts near your consumer unit. A current transformer clamp goes around the incoming grid feed cable to measure import and export. The diverter's output connects to your immersion heater's existing wiring. Total installation time: approximately two to three hours.
All-in installation costs in 2026 typically run as follows. Solic 200: £200-300 including hardware and installation. iBoost+: £280-380 all-in. myenergi Eddi: £350-450 all-in. These are approximate figures that vary by installer and region.
Annual saving from a diverter depends on how much surplus solar electricity you generate, your hot water usage, and your backup heating cost. A typical 3-bedroom household with a 3.5kWp solar system and a 200-litre hot water cylinder can expect to heat approximately 800-1,200 kWh of water per year from diverted solar electricity. At an avoided cost of 26p/kWh for electric water heating, that represents £208-312/year in saving. At an avoided cost of 6p/kWh for gas backup, the saving is £48-72/year.
Typical payback on a solar diverter with electric backup water heating: 1-2 years. With gas backup: 3-5 years. Both represent excellent return on a relatively modest investment. The payback is consistently one of the shortest of any home energy upgrade available to UK solar owners.
Combining solar diverter with Agile overnight for zero-cost hot water year-round
The most powerful setup for near-zero water heating cost combines a solar diverter during the day with cheap Octopus Agile overnight rates as the fallback.
In summer (April to September), the solar diverter runs during the day whenever surplus generation is available. On good days, the cylinder is heated entirely from free solar electricity by mid-afternoon. No gas or grid electricity is consumed for water heating at all. Total water heating cost: zero.
In winter (October to March), solar generation is insufficient to heat the cylinder from surplus alone on most days. The cylinder needs backup heating. Instead of allowing the gas boiler or a standard-rate electric immersion heater to do this, you schedule the immersion heater to run on the cheap Agile overnight window. Set the timer on your cylinder's immersion heater to run from 1:30am to 3:30am. At an Agile overnight rate of 3p/kWh and an immersion heater running at 3kW for two hours, the daily water heating cost is £0.18. Compare this to gas heating the same cylinder at 6p/kWh effective cost for the equivalent 6 kWh of heat: £0.36. The Agile immersion heater is cheaper than gas even in winter.
Check AgileAlert's overnight prices to see the current cheapest window in your region and time your cylinder's overnight heating accordingly.
The year-round result: your hot water costs approach zero. In summer, free solar. In winter, cheap Agile at 3p/kWh. A household spending £200-300/year on water heating before this setup can reduce that to £20-40/year. The double benefit, financial and environmental, is substantial: you are heating water with surplus wind at night and surplus sunshine by day, the two cleanest and cheapest energy sources available in the UK in 2026.
Compatibility with your hot water cylinder
Solar diverters require a property to have a hot water cylinder with an immersion heater. The immersion heater provides the resistive element that the diverted electricity heats. Without an immersion heater, there is nothing for the diverter to power.
Compatible setups include vented hot water cylinders with one or more immersion heaters, unvented hot water cylinders (mains pressure systems) with immersion heaters, and thermal store cylinders with immersion connections. Most UK homes built before the widespread adoption of combi boilers will have one of these configurations.
The setup that does not work with solar diverters is a combi boiler with no hot water cylinder. Combi boilers heat water on demand directly from the mains without storing it in a tank. There is no cylinder for the diverted electricity to heat. Households with combi boilers who want the benefits of solar water heating should consider a separate solar thermal system, or conversion to a system boiler with a cylinder, though both options are more expensive and disruptive than installing a simple diverter.
If you are unsure whether your property is compatible, look for a cylindrical tank in an airing cupboard or utility room. If it is there, you almost certainly have an immersion heater and a solar diverter will work. A qualified installer can confirm compatibility during a pre-installation survey, which most diverter installers offer for free or at low cost.