Is a Solar Diverter Worth It for Hot Water in 2026? (UK)
A solar diverter (Eddi, Solic, iBoost) costs £350–£900 fitted in 2026 UK. It pushes excess solar PV into your hot-water immersion instead of exporting it to the grid. Payback is 4–9 years if you have solar PV and an electric immersion in a hot-water cylinder. Not worth it if you only have a combi boiler (no cylinder), or if you''re on a 15p+ export tariff with low import cost. Here''s the maths.
What a solar diverter actually does
When your solar PV produces more electricity than the house is using, the surplus has only two homes: exported to the grid (for your SEG tariff rate), or wasted. A solar diverter sits between the consumer unit and the immersion heater in your hot-water cylinder, and continuously varies the power sent to the immersion to soak up exactly the surplus. It does this in 100ms steps, so the diverter chases cloud-on/off without flickering and without drawing from the grid.
Net effect: instead of exporting at 4–15p/kWh you store the energy as hot water for free use later. On a sunny day a 4kWp array typically produces 8–20 kWh of surplus, enough to fully heat a 200-litre cylinder. The most popular UK devices are myenergi Eddi (£530–£700 device), Solic 200 (£260–£330) and Marlec iBoost+ (£320–£430).
The 2026 payback maths
Assumptions: 4kWp solar PV array, UK average export tariff 6.5p/kWh, import tariff 27p/kWh, gas hot-water cost 8p/kWh. Annual surplus exported without a diverter: 1,800 kWh. Diverter cost: £650 fitted (mid-range Eddi install).
- Value of surplus exported (no diverter): 1,800 × £0.065 = £117/year
- Value of surplus diverted to hot water (displacing gas at 8p/kWh net of immersion efficiency loss): 1,800 × £0.075 = £135/year
- Net diverter benefit: £135 − £117 = £18/year — very poor payback if you''re displacing gas at a low rate
That above scenario is not worth it. But change the assumptions to all-electric hot water (no gas), displacing 27p/kWh import:
- Value of surplus diverted instead of exported: 1,800 × (£0.27 − £0.065) = £369/year
- Payback: £650 ÷ £369 = 1.8 years
That''s a no-brainer. The truth is in between for most homes — mixed gas/electric or partial year-round hot water demand — giving the typical 4–9 year payback.
How 2026 export-rate cuts changed the maths
A diverter earns its keep on the gap between what a surplus kWh is worth exported and what it saves heating water — and that gap moved in 2026. Octopus cut its popular Outgoing export tariff from 15p to 12p/kWh on 1 March 2026, while the best fixed SEG rates (OVO, E.ON Next) still sit around 15–16.5p/kWh and the flat Octopus SEG floor is roughly 4.1p/kWh. These are supplier rates set under the government Ofgem Smart Export Guarantee scheme. A lower export rate widens the divert-vs-export gap, so a diverter pays back faster on a low or mid export tariff than it did a year ago.
Worked 2026 example: a 4kWp array with an all-electric immersion and 1,800 kWh of annual surplus, on the 12p Outgoing tariff against a 27p import rate. Diverting that surplus to hot water is worth 1,800 × (27p − 12p) = £270/year versus exporting; on a £500 mid-range fitted install that is a 1.9-year payback. Drop to the 4.1p flat SEG floor and the gap widens to 22.9p, pulling payback under a year. Because of these rate cuts, 2026 installer estimates now put typical payback at 3–6 years for homes that currently export most of their generation — the shorter end of the older 4–9 year range. If you are weighing this against storage, see our solar diverter vs battery comparison, and EV owners should check whether an EV charger is worth it in 2026 before committing surplus to hot water alone.
When a solar diverter IS worth it
- You have solar PV and a hot-water cylinder with an electric immersion
- You''re on a low export tariff (4–7p/kWh)
- You''ve switched off your gas boiler in summer (April–September)
- You have high daytime surplus (typically 4kWp+ array, modest day-time consumption)
- Your cylinder is well insulated — an old uninsulated cylinder loses the heated water overnight
When a solar diverter is NOT worth it
- You only have a combi boiler — no cylinder to divert into. Don''t bother. A battery is a better fit.
- You''re on a high export tariff (12–16.5p/kWh fixed SEG, e.g. OVO or E.ON Next) with gas hot water — the gap is too small to repay the diverter
- Your PV array is under 3kWp — surplus too small to be meaningful
- You already have a battery — the battery soaks up surplus first; the diverter only sees scraps
- Your home is fully air-source heat pump with a hot-water cylinder — the heat pump is already 3–4× more efficient than direct immersion. Use surplus to charge a battery instead.
Solar diverter vs battery vs do-nothing
If you have surplus solar and the budget for one upgrade in 2026, the order of merit is usually:
- Battery storage (£5,500–£9,000 fitted, 7–12 year payback) — covers evening use, more flexible
- Solar diverter (£350–£900, 4–9 year payback) — cheap to fit, narrow benefit (hot water only)
- EV charger (£800–£1,500 fitted) — only adds value if you have an EV; solar-tariff chargers (Zappi, Ohme) work like a diverter for the car
If you already have a battery, a diverter usually doesn''t pay back. If you don''t have a battery and don''t want one, a diverter is a sensible £500–£700 add-on. For how it stacks up against home storage, see our solar battery cost guide.
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