Solar Immersion Diverters: Free Hot Water — or a Money-Loser?
The iBoost and Eddi turn surplus solar into hot water for £300–£650 fitted. But on a good export tariff, a diverter can quietly cost you money. Here’s the honest maths.
The short answer
A solar immersion diverter (the Marlec Solar iBoost+ and myenergi Eddi are the big two) senses when your panels are exporting and sends that surplus to your hot water cylinder’s immersion heater instead — free hot water from electricity you’d otherwise give away. Fitted for £300–£650, saving typically £100–£300 a year, with a 3–6 year payback. You need a hot water cylinder — combi homes can’t use one.
But here’s what most guides won’t tell you: the maths only works if your water heating costs more per kWh than your export tariff pays. Heat water with gas (~6–7p/kWh) while on a 12–16p export rate, and every diverted kWh loses you 5–9p. Heat water with grid electricity (~25p/kWh), and a diverter is a clear win. Do that one sum before you buy — we’ll walk you through it below.
What this guide covers
The immersion diverter is the cheapest self-consumption upgrade a solar home can make — a fraction of the price of a battery, fitted in an afternoon, no paperwork. For the right home it’s a quiet little money-printer. For the wrong home it’s a gadget that actively reduces your solar income. This guide is about telling those two homes apart.
How a solar immersion diverter works
The principle is elegantly simple. A small CT clamp sits on your electricity meter tails and watches, in real time, for the moment your panels generate more than your house is using — the point at which you’d normally start exporting. The diverter unit, wired between your consumer unit and your immersion heater, then feeds that surplus — and only the surplus — to the immersion element, modulating the power continuously to match it exactly. Kettle goes on? The diverter instantly backs off so you never buy grid electricity to heat water. Cloud passes? Same. The result: on a decent-sized system, a large share of your summer hot water heated entirely by electricity you’d otherwise have exported.
The requirements are modest but non-negotiable: solar PV that regularly exports a surplus, and a hot water cylinder (vented or unvented) with a standard 1–3kW immersion element. That second one rules out combi-boiler homes — no cylinder, nothing to heat. Installation is a same-day job for a Part P electrician, and unlike a battery there’s no DNO notification or approval needed.
The models: iBoost+ vs Eddi vs Solic 200
Three devices dominate the UK market in 2026, and choosing between them is mostly a question of budget and ecosystem:
| Device | Unit price | Fitted (typical) | Best for |
|---|---|---|---|
| Marlec Solar iBoost+ | ~£250–£330 | ~£350–£500 | Best value all-rounder |
| myenergi Eddi | ~£400–£549 | ~£500–£650 | App control & Zappi owners |
| Solic 200 | ~£150–£220 | ~£200–£300 | Tightest budget, no frills |
The Solar iBoost+ is the sensible default. The design has been stable for over a decade, most UK installers know it inside out, and it has a genuinely useful trick: it can wire to two immersion heaters and switch between them automatically. There’s no app and no cloud account — for plenty of households that’s a feature, not a limitation. It just sits in the airing cupboard and works.
The myenergi Eddi is the premium pick: colour display, Wi-Fi, full app monitoring, two controllable outputs — and, crucially, it talks to the rest of the myenergi ecosystem. Pair it with a Zappi EV charger and the two coordinate, prioritising your car over the water tank when the sun’s out. If you have (or plan) a Zappi, the Eddi’s premium makes sense; if not, you’re paying roughly £150 extra largely for the app.
The Solic 200 is the budget answer — no display, no app, just diversion. Perfectly adequate if the price gap matters more than monitoring.
The catch: do the export-tariff maths first
The one sum that decides everything
Every kWh you divert does two things at once: it saves you whatever that hot water would have cost to heat another way, and it forgoes the export payment you’d have received. So the rule is brutally simple: a diverter only makes sense if your water-heating cost per kWh is higher than your export rate per kWh. In 2026, with decent Smart Export Guarantee tariffs paying 12–16p/kWh, that sum fails for a lot of gas-heated homes — and nobody selling diverters is in a hurry to mention it.
Here’s how the maths lands for the common setups:
| Your water heating | Cost per kWh | vs 12–16p export | Verdict |
|---|---|---|---|
| Grid electricity (immersion) | ~24–25p | Saves 9–13p/kWh | Clear win |
| Gas boiler | ~6–7p | Loses 5–9p/kWh | Usually a loser |
| Gas boiler + low export rate (3–6p) | ~6–7p | Roughly break-even to small win | Marginal — switch tariff first |
| Heat pump (COP ~3) | ~8p effective | Loses on good tariffs | Rarely worth it |
The export landscape shifted this year, which changes the answer for many homes: Octopus cut Outgoing Fixed from 15p to 12p on 1 March 2026, while open-market rates like Ecotricity (~16p) and Good Energy (~15p) lead the flat tariffs — and the worst mainstream rates still languish at 3–4p. So the honest advice runs in a specific order: check your export rate first. If you’re on a poor one, switching tariffs is free and may earn you more than a diverter would save. If you’re stuck on a low rate, or you heat water electrically, then the diverter is excellent. (Your smart meter handles the export metering either way.)
Who should — and shouldn’t — buy one
A diverter is a great fit if: you have a hot water cylinder with an immersion element; your panels regularly export a surplus (typically 3kWp systems and up); and either you heat water with electricity, you’re off the gas grid, or your export rate is low (under roughly 6–7p/kWh). Households with good daytime hot water demand — families, home workers — squeeze the most from one.
Skip it if: you have a combi boiler (no cylinder — a battery or a good export tariff is your route instead); you heat water cheaply with gas while on a 12p+ export rate (you’ll earn more exporting); or a heat pump does your hot water — the heat pump already makes water at roughly a third of the electrical cost, so diverting raw kWh into a resistance element is a step backwards on a good tariff. And it should go without saying: with no solar panels there’s no surplus, so a diverter without solar does nothing at all.
Costs, savings & payback
Fully fitted, budget £300–£650 depending on device: the unit itself (£150–£549 across the three models) plus £120–£250 for a qualified electrician. Realistic savings for a well-matched home run £100–£300 a year — the top end for larger systems with electric water heating and strong daytime demand, the bottom end for smaller systems and shoulder-season sun. That puts payback at a very respectable 3–6 years on a device likely to last well beyond a decade — provided the tariff maths above works in your favour. It’s worth saying clearly: for the right home this is one of the best pound-for-pound returns in home energy; for the wrong home the payback is never, because the “saving” is negative.
Diverter vs battery: which surplus-soaker to buy?
Both solve the same problem — surplus solar leaving your house at a lower value than you could capture — but at wildly different scales. The diverter (£300–£650) converts surplus into exactly one thing, hot water, and saves £100–£300 a year. A home battery (£4,500–£7,000 installed) stores surplus for anything, covers your evenings, and can arbitrage cheap overnight rates — much bigger savings, roughly ten times the price. They also stack happily: many homes run both, letting the battery fill first and the diverter mop up what’s left on strong summer days. If you can only fund one and you have a cylinder plus a weak export rate, start with the diverter — it’s the cheapest self-consumption win there is. If you want whole-home flexibility, the battery is the bigger tool for a bigger job.
Solar immersion diverters: the honest summary
✓ The good
- Cheapest solar upgrade there is (£300–£650 fitted)
- Genuinely free summer hot water for the right home
- Same-day install, no DNO paperwork
- iBoost+ decade-proven; Eddi’s app & Zappi teamwork
- 3–6 year payback when the maths fits
- Stacks neatly alongside a battery
✗ The catches
- Loses money vs a 12–16p export rate if you heat water with gas
- Useless with a combi boiler — cylinder required
- Rarely worth it with a heat pump
- One trick only: hot water (a battery is far more flexible)
- Savings shrink in winter when surplus dries up
- Sellers rarely mention the export-tariff sum
Frequently asked questions
What is a solar immersion diverter and how does it work?
It’s a small unit that senses (via a CT clamp on your meter tails) when your panels are exporting, and feeds exactly that surplus to your hot water cylinder’s immersion heater, modulating power in real time so you never buy grid electricity to heat water. The best-known models are the Marlec Solar iBoost+ and myenergi Eddi. You need a cylinder with an immersion element.
Are they worth it in 2026?
Only if your water-heating cost per kWh beats your export rate. Electric water heating (~24–25p/kWh) vs any export rate: clear win. Gas (~6–7p/kWh) vs a good 12–16p export tariff: you’d lose 5–9p per diverted kWh — keep exporting. Best for electric hot water, off-gas homes, or anyone on a low export rate.
Solar iBoost vs myenergi Eddi — which is best?
The iBoost+ (~£350–£500 fitted) is the proven, no-app value pick and can switch between two immersion heaters. The Eddi (up to ~£650 fitted) adds a colour display, app monitoring, two outputs and Zappi EV charger integration. Choose iBoost+ for value, Eddi for the ecosystem. The Solic 200 (~£200–£300) is the budget option.
How much does one cost to install?
£300–£650 fully fitted: the unit (£150–£549 by model) plus £120–£250 for a Part P electrician, usually done in a day. No DNO notification needed. Typical savings of £100–£300/yr give a 3–6 year payback when the tariff maths works.
Can I use one with a combi boiler?
No — a diverter needs a hot water cylinder with an immersion element, and combis have neither. For combi homes, a home battery, a smart EV charger, or simply a strong export tariff are the better ways to use surplus solar.
Is a diverter better than a battery?
Different jobs: a diverter (£300–£650) makes hot water only, saving £100–£300/yr; a battery (£4,500–£7,000) stores surplus for any use, any time, saving more. Many homes run both. With a cylinder and a weak export rate, the diverter is the cheapest first step; for whole-home flexibility, it’s the battery.
Brilliant value — but only after you’ve checked one number
The bottom line: At £300–£650 fitted, a diverter is the cheapest solar upgrade going — free summer hot water and a 3–6 year payback for the right home. But check the one number first: if your export tariff pays more per kWh than your water heating costs (gas homes on 12–16p rates, most heat-pump homes), diverting loses you money and you should keep exporting. Electric hot water, off-gas homes and low-rate exporters: buy with confidence — the iBoost+ for value, the Eddi for the app and Zappi teamwork.
Get my free quotes →The verdict
The immersion diverter earns its reputation as the cheapest self-consumption upgrade in home solar — a £300–£650 device that quietly turns exported sunshine into a free tank of hot water, fitted in an afternoon with no paperwork. The Solar iBoost+ is the proven value pick, the myenergi Eddi the smarter choice for app lovers and Zappi owners.
But it’s not for everyone, and we’d rather tell you that plainly: if you heat water with gas on a good 12–16p export tariff, or a heat pump does your hot water, the maths says keep exporting — and if you have a combi boiler, there’s nothing for it to heat. Where a diverter shines is electric hot water, off-gas homes and low export rates: there, it’s one of the best pound-for-pound returns in home energy. The smart move is to have the whole system specced together — panels, battery, diverter, tariff — by a good MCS-certified installer who’ll tell you which pieces your home actually needs. Start with a couple of free, no-obligation quotes and ask them to run your export-tariff sum.
Important
Going Solar UK is an introduction service, not an installer or financial adviser. This guide is general 2026 information, not financial advice. Prices, export tariff rates, savings and payback periods vary by property, system, supplier and over time — Smart Export Guarantee rates in particular can change with around 30 days’ notice, so always confirm live rates and get a tailored assessment before buying. Savings figures are typical estimates, not guarantees.
We may receive a commission when we introduce you to an installer, at no cost to you. Any contract for work is directly between you and the installer.