Case study · Berkshire
Install completeThe kit was right. The order was the regret.
This home put solar on the roof first. A year later they added a battery, and it did roughly three times the financial work of the panels. Here is the whole story in four numbers, then how we got there.
Saving / yr
Battery vs solar
Year payback
Net by year 10
Figures modelled from the home's own half-hourly meter data against a published dual-rate tariff. Indicative; your home's numbers will be different.
Where the saving comes from
The battery does roughly 3× the work of the solar.
The panels earn from what leaves the house; the battery earns from what stays, by shifting cheap overnight power into peak hours. Modelled on the home's own meter data.
The starting state
An 8 kWp array earning its modest export return. The other half of the system was still unbuilt.
8 kWp on the roof.
Generates, the household self-consumes some and exports the rest at the SEG rate. A 19th-century five-bed in RG8, gas heating, all-electric loads.
~£415 a year from export.
2,766 kWh exported in 2025 at 15p/kWh. Real, but capped at whatever leaves the house in daylight.
Most of the load is overnight.
Solar can’t reach it. The household had no model of what a battery on the cheap overnight window could earn against the same load.
Solar is one half of the system. The battery is the other half, and it does more of the work.
What we modelled
Solar alone, against solar plus a 30 kWh battery on night-charge dispatch.
| Configuration | Solar onlyThe starting state, 2025 | Recommended · chosen installSolar + 30 kWh batteryThree modules · night-charge |
|---|---|---|
| Battery | – | 30 kWh |
| Battery cost | – | £8,444 |
| Annual saving | ~£415SEG export only | £1,798~3× the solar's own earnings |
| Payback | – | 5.5 yrs |
Where the saving comes from
The arbitrage
The battery earns from what stays in the house, not from what leaves it.
The 10-year view
~£9,500 net by year 10, and that's the cautious read.
Payback
0.0years
Warranty
0years
Net, to year 10
~£0
Why these numbers hold
17,520 readings, not a guess.
A full year of the home’s actual half-hourly meter data, not a profile or a postcode average.
A real, published tariff.
A dispatch model run against Octopus’s published Jan-26 dual-rate tariff; every charge and discharge costed.
No stake in the answer.
We sell no hardware and take no installer commission. Where the numbers favoured a smaller install, we said smaller.
The fine print: strategy, the inverter limit and the 10-year caveat
We model money and carbon and present the trade-off. Night-charge returned £122/yr more here; solar-first cut ~67 kgCO2/yr more. The financial line was their priority; the call was theirs.
The 7.2 kW inverter caps overnight charging at ~43 kWh per six-hour window. A larger battery would not earn more, because it could not be filled cheaply in time. Three modules is the sweet spot, not four or five.
The 10-year figure is where the warranty ends, not the battery. Systems of this class commonly run 12 to 20 years; any saving past year 10 is unmodelled upside.
The takeaway
“We started with solar as that's what we've heard about in the papers. I didn't quite realise how suited our house was to a battery set up as well.”
Verified customer · RG8 19th-century five-bed · install completed Feb 2026
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Curious what the numbers look like for your home?
This was modelled for one specific home. Yours has its own load, tariff and roof, so the answer will be different. We work it out from your own data, and we don't sell the kit.