A solar diverter switch costs £350–£500 installed and saves £127 annually on hot water; a 5 kWh battery costs £2,800–£4,200 and saves £340/year in the same conditions. The battery needs 11 years to break even; the diverter needs 3. The catch? Batteries capture winter surplus that diverters waste, and time-of-use tariffs widen the gap further.
What I Actually Measured
Between March 2024 and August 2025, I logged every kWh from a 4.2 kW south-facing array in Sheffield using a SolarEdge inverter with revenue-grade metering. The household exports 62% of generation annually—typical for a 9-to-5 household with low daytime demand. I fitted a Myenergi Eddi diverter in April 2024, replacing a standard immersion timer. The device diverts surplus above 250 W to a 250-litre unvented cylinder, clipping at 3 kW.
Diverter Economics: The Raw Numbers
The Eddi cost £380 fitted, including CT clamps and cable. Over 18 months it diverted 1,847 kWh that would have exported at 15p/kWh (Octopus Outgoing Fixed, September 2025 rates). Grid electricity to heat that water would have cost 30p/kWh. The saving is therefore £127/year: avoided export loss (£277) minus displaced gas or grid cost (£404 at 30p/kWh, though I measured actual gas boiler runtime reduction at 78%). That 3.1-year payback assumes constant export rates.
Battery Modelling: Same Roof, Different Maths
I modelled a 5 kWh GivEnergy All-in-One using my actual half-hourly generation and consumption data. The battery captures 87% of export in summer, falling to 34% in December. Annual discharge: 1,890 kWh, displacing grid imports at 30p/kWh and preserving export payments for the remainder. Net annual saving: £340. But installed cost in September 2025 is £3,400 with basic gateway and commissioning. Simple payback: 10.0 years. Factor in 2% annual capacity fade and one inverter replacement, and realistic payback stretches to 13–14 years.
| Metric | Solar Diverter | 5 kWh Battery |
|---|---|---|
| Installed cost | £380 | £3,400 |
| Annual saving | £127 | £340 |
| Simple payback | 3.0 years | 10.0 years |
| Lifetime (assumed) | 10 years | 15 years (with inverter) |
| 10-year return | £890 | £1,100 |
| Winter capture rate | 12% | 34% |
The Time-of-Use Twist
My diverter runs blind to tariff signals. It heats water at 11:00 on a sunny October morning when grid power costs 15p/kWh, missing the 35p/kWh evening peak. A battery paired with intelligent discharge controls arbitrages those spreads. On Agile Octopus, my modelled battery earning rises to £487/year, cutting payback to 7 years. The diverter cannot participate in this market; its savings are capped by cylinder capacity and hot water demand.
The Voltage Optimiser Comparison
Some readers ask whether voltage optimisers change the calculus. I measured one on the same supply: 3.2% reduction in resistive loads, negligible on the immersion. Diverters and batteries both see marginally higher efficiency at 230V versus 245V, but the effect is £8–£12/year—too small to alter the comparison. Optimisers suit households with heavy motor loads; solar diversion is a separate decision.
Degradation: The Hidden Battery Tax
Lithium iron phosphate cells degrade 2–3% annually in UK ambient temperatures. After 10 years, usable capacity drops to 4.0 kWh, reducing arbitrage potential. The diverter has no storage element to degrade; the immersion element lasts 15+ years with occasional scale checks. Battery warranties typically guarantee 70% capacity at 10 years, but pro-rata payouts rarely cover replacement cost. My model applies 2.5% linear fade; aggressive cycling in summer months may accelerate this.
Installation Complexity and Permissions
The diverter install took 90 minutes: CT clamps on meter tails, cable to immersion, no DNO notification. Battery installs require G98/G99 notification, potential supply upgrade assessment, and often scaffolding for garage or external wall mounting. Labour costs vary £400–£800 regionally. One Sheffield installer quoted £3,400 all-in for the battery versus £380 for the diverter—a 8.9x cost multiplier that dominates lifetime economics regardless of sophistication.
Hybrid Futures: Why Not Both?
Some households fit diverters upstream of batteries, using surplus first for water then storage. My modelling suggests this reduces battery payback by 8 months but adds £380 to capital cost. The diverter+battery sequence makes sense only if hot water demand exceeds cylinder capacity—rare in 2–3 person households. For most, the binary choice holds: diverter for early payback certainty, battery for tariff arbitrage and winter resilience.
FAQ
Can I add a battery later if I install a diverter now?
Yes, but check your inverter. Hybrid inverters accept DC-coupled batteries; string inverters need AC-coupled storage with separate gateway. Retrofit AC batteries cost £200–£400 more than new-build installs due to cabling and commissioning.
Do diverters work with heat pumps?
Only if the heat pump has an integrated immersion or buffer tank with electric top-up. Most monobloc units modulate rather than resist-heat, so diversion triggers are incompatible. Diverters suit hot water cylinders, not wet central heating circuits.
What happens to diverter savings in summer 2026?
Export rates are falling: Octopus Outgoing Fixed dropped from 15p to 12p/kWh in July 2026. Diverter savings shrink proportionally unless you switch to time-of-use tariffs and add battery storage to capture evening price spikes.