A solar power diverter only makes you money if the heat it replaces costs more than the export payment it costs you. Under the Ofgem price cap running from 1 October to 31 December 2026, electricity is 26.32p per kWh and gas is 7.97p per kWh. A typical export rate is around 12p. So if your hot water comes from an immersion heater, diverting a unit is worth about 14p to you. If it comes from a gas boiler, diverting that same unit saves at least 7.97p (a little more once boiler losses are counted) and gives up 12p, and you are up to about 4p worse off every time it runs.
That is the whole decision, and almost nobody selling a diverter puts it that way. The devices are good. The engineering is sound, they are cheap next to a battery, and in the right house they are one of the best value things you can bolt onto a solar array. In the wrong house they are a gadget that costs you money quietly, every sunny afternoon, while telling you on an app how much you are saving.
Every figure below was checked against the live source on 24 August 2026, and the price cap figures were updated to the current cap on 10 October 2026.
What is a solar power diverter?
It is a controller that watches the flow at your electricity meter and, when it sees you exporting, sends that surplus to your immersion heater instead. A clamp on the meter tail detects the export, the controller modulates the power going to the immersion element to match it, and the surplus that would have gone out to the grid heats your hot water cylinder instead.
Two products come up again and again on UK quotes. myenergi's eddi, which calls itself "the market's leading solar power diverter", is sold as a device that "redirects renewable energy to your immersion heater, storage heater, underfloor heating or electric heater", handling any resistive load up to 3.68kW, with a minimum load of 100W and a 3W standby draw. Marlec's Solar iBoost+ works the same way, with a sender and clamp at the meter signalling wirelessly to a control unit next to the cylinder, and claims over 150,000 UK homeowners.
Note what a diverter is not. It is a load on your side of the meter, not extra generation. It does not increase what your array produces, and it does not change the size of the system your installer applies to connect. That paperwork is covered in our guide to G98 and G99 solar connections.
Does a solar diverter actually save money?
Only when the energy it displaces is dearer than the export you give up. Every kWh you divert is a kWh you do not export, so it is a kWh you are not paid for. That forgone payment is the real price of the "free" hot water.
Take the three numbers as they stand today. The Ofgem price cap for 1 October to 31 December 2026 puts electricity at 26.32p per kWh and gas at 7.97p per kWh for a direct debit customer. Gas includes 5% VAT; there is no VAT on electricity from 1 October 2026 to 31 March 2027. On the export side, Octopus currently pays 12p per kWh flat on Outgoing Octopus, with its Prime tariff paying 16p between 4pm and 7pm and 9p the rest of the day. The full spread across suppliers is in our Smart Export Guarantee rates comparison.
| How you heat water now | Value of 1 kWh diverted | Export payment given up | Net per kWh |
|---|---|---|---|
| Immersion heater or electric only | 26.32p | 12p | +14.32p |
| Gas boiler, 12p export | 7.97p or more | 12p | -4.03p or better |
| Gas boiler, 3p export | 7.97p or more | 3p | +4.97p or more |
| No export payment at all | 7.97p to 26.32p | nothing | All of it |
The "7.97p or more" needs saying out loud. That figure assumes your boiler turns every unit of gas into a unit of hot water, which no boiler does. Real gas heat costs you more than 7.97p per useful kWh, so the gas rows above understate what a diverted unit saves. Even allowing for boiler losses, the case stays thin or negative at a 12p export rate.
Two situations flip the maths completely in the diverter's favour. The first is having no export payment: plenty of older arrays were never registered for anything, and if nobody is paying you to export, the surplus is worth precisely nothing until you use it. The second is being an existing Feed-in Tariff generator on deemed export. Deemed export pays you for an assumed proportion of what you generate rather than a metered reading, so what you actually send to the grid does not change the payment, and a diverted unit costs you nothing at all. Our guide to SEG versus the Feed-in Tariff explains which side of that line you are on.
How much does a solar power diverter cost in the UK?
The hardware starts at £439 including VAT for a single-phase myenergi eddi, with its listed price range running up to £779, and you need an electrician on top unless you are competent to wire it yourself. Those are the manufacturer's own listed prices on its UK shop, checked on 24 August 2026. myenergi says installation "can take less than an hour" and recommends consulting an electrician, and the unit ships with the current transformer and 5m cable it needs, plus a three year warranty.
The VAT position is worth understanding before you compare prices, because it does not work the way people assume. HMRC's VAT Notice 708/6 added smart diverters to the temporary zero rate at section 2.20: "From 1 February 2024, the retrofitting of a smart diverter to one or more of the microgeneration systems (listed in section 2.7) installed in residential accommodation or buildings intended for use solely for a relevant charitable purpose qualifies for the temporary zero rate." But that relief attaches to the installation, not the box. The same notice is blunt about buying one over the counter: "if you supply energy-saving materials without installing them your supply will be standard-rated. For example, the sale of energy-saving materials by a retailer is always standard-rated."
So a retail price includes 20% VAT, while the identical unit supplied and installed by a business is zero-rated until 31 March 2027, after which section 2.1 confirms it reverts to the 5% reduced rate. Compare an installed quote against the DIY price plus an electrician's day rate, not against the sticker alone. The same rule governs panels and batteries, and we set the whole thing out in VAT on solar panels in the UK.
Break-even is then simple division. At the 14.32p per kWh an immersion-heated home gains, a £439 unit pays for itself after roughly 3,100 kWh diverted. At the 4.97p or more a gas home on a 3p export rate gains, the same unit needs up to about 8,800 kWh through it, which is a different conversation entirely.
How much hot water will it actually make?
Around 7.9 kWh of surplus reheats a typical 150 litre cylinder from cold, so most homes are talking about one tank on a good day, not endless free hot water. The arithmetic is fixed by physics: raising 150 litres from 15°C to 60°C takes about 7.9 kWh. At a 3kW immersion element running flat out that is a shade over two and a half hours, though a diverter never runs flat out for long, because it modulates down to match whatever surplus is actually there minute by minute.
Put a price on that tank at today's rates. Heating it by immersion off the grid costs £2.08. Heating it with gas costs at least 63p, assuming a boiler that is 100% efficient (a real boiler costs more). Exporting the same 7.9 kWh at 12p would have earned you 95p. The gas household therefore loses up to about 32p on every tank the diverter heats, and the immersion household gains about £1.13.
Whether you have that much surplus to play with in the first place depends on system size and how much you use during the day, which is the same self-consumption question that drives payback on the array itself. We work through it in are solar panels worth it in the UK.
Diverter or battery?
They solve different problems, and the diverter is not a cheap battery. A battery stores surplus as electricity you can use for anything later, displacing units you would otherwise buy at 26.32p. A diverter converts surplus into heat you can only use as hot water, displacing whatever your water heating costs. In a gas home those two numbers are 26.32p and a little over 7.97p (the gas price plus boiler losses), which is why a battery can stack up in a house where a diverter does not.
The counterweight is price. The Energy Saving Trust's battery storage guide, last updated 6 October 2026, puts home batteries at £1,500 to £10,000 with a 5kWh system at around £4,600, against £439 for diverter hardware. If your hot water is electric, a diverter is usually the first thing to fit and the battery is the second. Our guide to solar battery storage costs and payback covers the other half of that decision.
A diverter is not solar water heating
Solar water heating, or solar thermal, is a separate technology with its own panels, and it costs roughly ten times what a diverter does. The Energy Saving Trust describes solar thermal as roof-mounted collectors circulating a water and antifreeze mix through a coil in your cylinder, needing around five square metres of roof, and puts the cost of a typical installation at around £5,300. It reckons such a system supplies around 90% of hot water in summer, dropping to around 25% in winter, and roughly half across the year. Those figures come from its solar water heating advice page, last updated 20 May 2026.
If you are choosing between them and your roof can take PV, the Energy Saving Trust's own steer is telling: "If your roof is suitable for solar collectors, it's worth considering a solar panel system for free, renewable electricity instead." A PV array plus a diverter gives you electricity first and hot water from the leftovers. Solar thermal gives you hot water only.
Hot water safety: do not let the tank sit lukewarm
A cylinder that is warmed to bath temperature and never taken up to 60°C is sitting in the range where legionella bacteria multiply. HSE guidance sets the reference points plainly: hot water storage cylinders "should store water at 60°C or higher", hot water "should be distributed at 50°C or higher", and cold water below 20°C. The bacteria do not survive above 60°C and are dormant below 20°C, with multiplication happening between 20°C and 45°C.
That guidance carries legal duties for landlords and workplaces rather than for an owner occupier, but the temperatures are the standard the whole industry works to, and they are the reason a diverter should not be your only source of water heating. Keep the boiler or immersion doing a regular full-temperature cycle, and treat the diverted solar as the thing that reduces how often it needs to run rather than the thing that replaces it. The eddi's boost timer exists precisely for this, letting you schedule a top-up at times of low generation.
What to ask before you fit one
- What heats my water today, and what does that heat cost per kWh? This one answer decides the whole thing.
- What export rate am I on, or could I move to? A 3p rate and an 18p rate produce opposite conclusions from the same hardware.
- Am I on deemed export under the Feed-in Tariff? If so, diverting costs you nothing in lost payments.
- Is the quote supply and install, and is it zero-rated? A retail unit carries 20% VAT that an installed one does not, until 31 March 2027.
- Does my cylinder have a working immersion heater and a spare fused switch? No cylinder, no diverter. Combi boiler homes have nowhere to put the heat.
If you are still specifying the array itself, get the diverter priced as part of the job rather than as an afterthought, since it is zero-rated inside an installation and standard-rated bought alone. Our guide to choosing a solar panel installer covers how to check MCS certification and consumer-code membership first, and you can request quotes through RenewQuote to start comparing. The full cost picture for the system it bolts onto is in solar panel costs in the UK.
Solar power diverters: frequently asked questions
Is a solar diverter worth it in the UK?
It depends entirely on how you heat water now. If your hot water comes from an immersion heater or you are all-electric, a diverted unit saves you the 26.32p per kWh electricity costs under the Ofgem cap for 1 October to 31 December 2026 and gives up around 12p of export payment, so you gain roughly 14p per kWh and a £439 unit pays back after about 3,100 kWh. If your hot water comes from a gas boiler, that same unit saves at least 7.97p of gas (a little more once boiler losses are counted) and still gives up 12p of export, which on a 12p export rate leaves you up to about 4p worse off on every unit diverted. Homes with no export payment at all, and Feed-in Tariff generators on deemed export, gain the full value either way.
How much does a solar power diverter cost?
myenergi lists its eddi from £439 including VAT for the single-phase unit, with its listed range running up to £779, as shown on its UK shop on 24 August 2026. Installation is extra unless you wire it yourself, and myenergi says the job can take under an hour but recommends consulting an electrician. Retail prices include 20% VAT. Where a business supplies and installs the diverter it is zero-rated under VAT Notice 708/6 section 2.20 until 31 March 2027, so an installed quote should be compared against the retail price plus an electrician rather than against the sticker price alone.
Does a solar diverter reduce my Smart Export Guarantee payments?
Yes, and that is the cost people forget. SEG is paid on metered export, so every unit the diverter sends to your immersion heater is a unit your supplier does not see and does not pay for. At a 12p export rate, diverting the 7.9 kWh needed to reheat a 150 litre cylinder costs you about 95p in forgone payments. The exception is an existing Feed-in Tariff generator on deemed export, who is paid on an assumed proportion of generation rather than a meter reading, so diverting does not reduce that payment.
How much hot water can a solar diverter heat?
Roughly one cylinder on a good day. Raising 150 litres of water from 15°C to 60°C takes about 7.9 kWh, which at a 3kW immersion element is a little over two and a half hours at full power. In practice a diverter never runs at full power for long, because it modulates the load to match the surplus available minute by minute, so a full tank is the product of a whole sunny day rather than an afternoon. Winter generation will rarely produce enough surplus to matter, which is why a diverter supplements your normal water heating rather than replacing it.
Is a solar diverter better than a battery?
They do different jobs. A battery stores surplus as electricity you can spend on anything later, displacing units you would otherwise buy at 26.32p per kWh under the current cap. A diverter turns surplus into heat you can only use as hot water, displacing whatever your water heating costs, which in a gas home is at least 7.97p per kWh, a little more once boiler losses are counted. That is why a battery can make sense in a house where a diverter does not. A diverter is also far cheaper: the Energy Saving Trust puts battery storage at £1,500 to £10,000, with a 5kWh system around £4,600, against £439 for diverter hardware. In an all-electric home the diverter is usually the first thing to fit and the battery the second.
Can I fit a solar diverter to a combi boiler?
Not usefully, because there is nowhere to put the heat. A diverter works by sending surplus electricity to an immersion heater inside a hot water cylinder, and a combi boiler heats water on demand without storing it. No cylinder means no immersion element and no store to warm up. myenergi does note that an eddi can work alongside a gas boiler in homes that do have a cylinder, using its boost timer to let the boiler heat water at times of low generation, but the cylinder itself is the part you cannot do without.
Is a solar diverter the same as solar water heating?
No. Solar water heating, also called solar thermal, uses its own roof collectors circulating a water and antifreeze mix through a coil in your cylinder, and the Energy Saving Trust puts a typical installation at around £5,300, needing around five square metres of roof. A diverter has no panels of its own. It simply redirects surplus electricity from a photovoltaic array you already have to an immersion heater you already have, starting at £439 for the hardware. The Energy Saving Trust's own advice is that where a roof suits solar collectors, it is worth considering a solar panel system for free renewable electricity instead.
Do I need to set my cylinder to 60°C if I have a diverter?
You should still get the cylinder up to full temperature regularly rather than leaving it lukewarm. HSE guidance sets the reference standard: hot water storage cylinders should store water at 60°C or higher and hot water should be distributed at 50°C or higher, because legionella bacteria do not survive above 60°C but multiply between 20°C and 45°C. Those duties apply to landlords and workplaces rather than owner occupiers, but the temperatures are the standard the industry works to. Treat diverted solar as the thing that reduces how often your boiler or immersion runs, not as a replacement for a full-temperature cycle.
All figures and quotations on this page were verified against the live primary sources on 24 August 2026, with the price cap figures re-checked on 10 October 2026: Ofgem's price cap unit rates and standing charges for 1 October to 31 December 2026; HMRC VAT Notice 708/6 on energy-saving materials, sections 2.1, 2.7 and 2.20; HSE guidance on legionella control in hot and cold water systems; the Energy Saving Trust's solar water heating advice, last updated 20 May 2026; and the manufacturer listings for the myenergi eddi and Marlec Solar iBoost+. Prices, export rates and the price cap change, so check the source before relying on a figure. The price cap is reviewed quarterly; the current level runs 1 October to 31 December 2026 and the next takes effect on 1 January 2027.