Is a Solar Diverter Worth It for Hot Water in 2026? (UK)

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.

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 15p+ export tariff (e.g. Octopus Outgoing Fixed) 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:

1. Battery storage (£5,500–£9,000 fitted, 7–12 year payback) — covers evening use, more flexible
2. Solar diverter (£350–£900, 4–9 year payback) — cheap to fit, narrow benefit (hot water only)
3. 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.

FAQs

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