How to Plan a Solar Diverter for Hot Water in 2026 (UK)
A solar PV diverter (also called an immersion optimiser or PV diverter) takes the surplus electricity your solar panels would otherwise export to the grid and sends it to your hot-water cylinder's immersion heater — heating a full tank of water for free. This 2026 step-by-step guide walks through how a diverter works, the leading devices (myenergi Eddi, Marlec Solar iBoost+, Solic 200), who it suits, the realistic £90–£200/year savings, and how it interacts with your SEG export tariff so you don't accidentally make yourself worse off.
How do I plan a solar diverter for hot water?
You need a hot-water cylinder with a spare immersion heater and a solar PV system that regularly exports surplus power. If you have both, a diverter is almost always worth it. The plan:
- Confirm you have a vented or unvented cylinder with an immersion (combi-only homes can't use one)
- Estimate your daytime PV surplus (the export you're giving away)
- Choose a single- or dual-load device (myenergi Eddi, iBoost+, Solic 200)
- Check consumer-unit space and where the CT clamp will sit
- Budget £300–£600 for the device plus £150–£300 for a qualified electrician
- Commission it, set the boost timer, and monitor for the first month
Typical saving: £90–£200/year on water heating. Payback: 3–6 years. Not suitable for: combi-boiler homes with no cylinder.
What a solar diverter actually does
Without a diverter, any solar electricity your home isn't using in real time flows out to the grid as "export". On a Smart Export Guarantee (SEG) tariff you're typically paid somewhere between 1p and 15p per kWh for that export, depending on supplier — and a lot of homes are on the lower end. A solar diverter intercepts that surplus before it leaves the property and routes it into your immersion heater instead, so the energy heats your hot-water cylinder rather than being sold cheaply.
The mechanism is a current transformer (CT) clamp fitted around the meter tails (or the main incoming line). The CT clamp continuously senses the direction and amount of power flow. The moment it detects export — i.e. you're generating more than the house is consuming — the diverter modulates the immersion heater, ramping its power draw up and down many times a second to exactly soak up the surplus. This is why a good diverter trickles power into the tank smoothly rather than switching a fixed 3 kW element fully on and off: it heats the water using only the spare solar, never pulling expensive grid electricity to do so.
Once the cylinder thermostat reaches its target temperature, the diverter stops and any further surplus goes back to export as normal. Most units also include a manual or timed boost so you can top the tank up from the grid on a dull day or overnight on a cheap off-peak rate. Some support a second load — a second immersion, a towel rail, or in newer homes an EV or storage heater — so surplus is prioritised across multiple destinations.
The Best Solar Diverters in 2026
Three devices dominate the UK market in 2026. They all do the core job — diverting surplus to an immersion — but differ on multi-load support, monitoring and price. Indicative device-only prices (before installation):
Prices are device-only and indicative for Q2 2026; add £150–£300 for installation by a qualified electrician. The myenergi Eddi commands a premium for its app, build quality and the ability to slot into a wider Zappi (EV) and Libbi (battery) setup later. The iBoost+ uses a neat wireless sender so you don't need a wired CT run back to the unit. The Solic 200 is the budget choice — no screen, no app, it just works in the background.
Compatibility & Eligibility
A solar diverter only makes sense if you have somewhere to put the heat. The single biggest determining factor is whether you have a hot-water cylinder.
You'll benefit if…
- You have a vented or unvented hot-water cylinder
- The cylinder has a working immersion heater (or a spare boss for one)
- You have solar PV that regularly exports during the day
- Someone is home in the day, or you can boost on a timer
- Your SEG export rate is low (so keeping the energy is worth more)
- You have PV with or without a battery — both work
A diverter won't help if…
- You have a combi boiler with no cylinder — there's no tank to heat
- Your PV is tiny (under ~2 kWp) and rarely exports
- You're on a very high SEG export rate (export may pay more than the gas/heat you'd offset)
- Your cylinder is already heated cheaply by a heat pump on a smart tariff
Combi homes: consider a battery instead — it stores surplus for any appliance, not just hot water.
How to Plan a Solar Diverter: 6 Steps
Check you have a cylinder + spare immersion
Find your hot-water cylinder (airing cupboard, loft or garage) and confirm it has an immersion heater fitted, or a spare immersion boss. No cylinder means no diverter — combi-only homes should look at a battery instead. Note whether it's vented or unvented; both are fine.
Size your PV surplus
Look at your generation/export data (inverter app or SEG export readings). A typical 4 kWp array exports several kWh on a sunny day — more than enough to heat a 120–210 litre cylinder. If you rarely export because the house consumes everything, the saving will be smaller.
Choose a single- or dual-load device
One immersion = a single-load unit (Solic 200, iBoost+) is plenty. If you want to also feed a second immersion, towel rail or future EV charger, pick a dual-load unit like the myenergi Eddi. Match the device to what you'll realistically connect over the next few years.
Check consumer-unit space & CT placement
The diverter needs a spare way in your consumer unit (or a dedicated fused spur) and a CT clamp around the meter tails or main incomer, positioned to read total grid import/export. If your meter cupboard is cramped or far from the cylinder, flag this to your electrician early — wireless-sender units (iBoost+) help here.
Budget the device + installation
Budget £300–£600 for the device and £150–£300 for fitting by a qualified electrician (or MCS-registered installer if bundled with a new PV system). All-in, most diverters land at £450–£900 installed. Get the install added when other electrical work is happening to save on call-out costs.
Commission & set timers/boost
After install, your electrician confirms the CT reads the right direction and the immersion modulates on export. Set the cylinder thermostat to a safe 60°C+ for Legionella control, configure any boost timer for dull days, and monitor for the first month to confirm it's diverting surplus rather than pulling grid power.
Realistic Savings & Payback in 2026
A solar diverter typically saves a UK household £90–£200 a year on water heating, depending on PV size, cylinder size, occupancy pattern and what you'd otherwise use to heat the water. With an installed cost of roughly £450–£900, that's a payback of about 3–6 years — and the unit then runs effectively for free for the rest of its 10–15 year life.
These figures assume you'd otherwise heat that water using grid electricity or gas. If your cylinder is normally heated by a cheap off-peak immersion or an efficient heat pump, the marginal saving is smaller — see the SEG section below before you commit.
How a Diverter Interacts With Your SEG Export Tariff
This is the part most guides skip. A diverter keeps energy you would otherwise export — so it reduces your SEG export income. Whether that's a net win depends entirely on your export rate versus the value of the heat you're displacing.
The simple test: compare the value of 1 kWh of hot water against your SEG export rate.
- Low SEG rate (1–5p/kWh): divert away — keeping the energy as hot water (worth ~7p+ in displaced gas, or 24.7p+ if you'd otherwise use a grid immersion) easily beats the export pennies.
- Mid SEG rate (5–10p/kWh): still usually worth diverting if you'd otherwise heat water with grid electricity; closer to neutral if you heat with cheap mains gas.
- High SEG rate (12–15p/kWh): do the maths carefully. If your gas is cheap, exporting at 15p and heating water on gas may pay more than diverting. A battery or self-use of higher-value loads may beat a diverter here.
A practical rule of thumb for 2026: with the electricity unit rate around 24.7p/kWh (Q2 2026 cap) and most SEG export tariffs paying well under that, diverting surplus into hot water you'd otherwise heat with the grid is almost always the better outcome. The case is closest to neutral only for homes on a high export rate who heat water with cheap gas.
Part P, Electrical Safety & Legionella
A solar diverter is a permanent electrical installation wired into your consumer unit, so it falls under Part P of the Building Regulations in England & Wales. The work must be carried out by a qualified electrician — ideally one registered with a competent-person scheme (NICEIC, NAPIT or similar) who can self-certify the work, or it must be notified to building control.
- Never DIY the consumer-unit connection. CT placement and live tails work is dangerous and notifiable — always use a qualified electrician.
- Keep the cylinder thermostat at 60°C or above. Solar-heated water can sit lukewarm; running the immersion to 60°C+ at least periodically controls Legionella bacteria.
- Confirm the CT direction at commissioning. A backwards CT clamp will pull expensive grid power instead of using surplus — get it verified before sign-off.
- Check unvented cylinder G3 compliance. Unvented cylinders have their own safety requirements; the diverter must not interfere with the cylinder's thermal cut-outs.
When a battery or heat pump beats a diverter
A diverter only ever does one thing: turn surplus solar into hot water. That's efficient and cheap, but it caps your benefit at the value of your hot-water demand. If your cylinder is already up to temperature by lunchtime on a sunny day, every extra kWh of surplus goes back to export anyway.
A home battery stores surplus for any evening load — lighting, cooking, TV, an EV — so it captures more value from a large array, though at a far higher cost (£4,000+ installed). For homes with big surplus and high evening usage, a battery often delivers more total saving than a diverter. Many households fit both: divert to hot water first, then store the rest.
If you're replacing your heating, an air-source heat pump on a smart tariff can heat both your home and water far more efficiently than a resistive immersion (a heat pump delivers roughly 3 kWh of heat per 1 kWh of electricity). In that case a diverter adds little — the heat pump already heats the cylinder cheaply. The diverter shines specifically for homes keeping a conventional cylinder and immersion, where it's the cheapest way to stop giving surplus away.
Get Solar & Installer Quotes — Free
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Our sources for this guide
Device pricing and specifications are cross-referenced against current 2026 manufacturer data; regulatory and tariff references against primary UK sources.
- myenergi — Eddi solar power diverter
- Marlec — Solar iBoost+
- Solic 200 — solar immersion controller
- Ofgem — Smart Export Guarantee (SEG)
- gov.uk — Part P (electrical safety) Approved Document
- HSE — Legionella and hot-water systems
Links open in a new tab on external sites. We do not benefit commercially from any of these links; they are included to help readers verify claims and research further. If you spot a broken or outdated link, email info@bestbuilders.co.uk.