The standard model
Payback = installed cost ÷ annual savings. Annual savings = self-consumed solar × import price + exported solar × export rate.
For a 4 kWp system in West Sussex at £7,500 installed, generating 4,100 kWh/year, with 35% self-consumption at 28p/kWh and 65% export at 8p/kWh: annual saving = 4100 × 0.35 × 0.28 + 4100 × 0.65 × 0.08 = £402 + £213 = £615/year. Payback ≈ 12.2 years.
Add a 10 kWh battery (£6,500) that lifts self-consumption to 80%: annual saving = 4100 × 0.80 × 0.28 + 4100 × 0.20 × 0.08 = £918 + £66 = £984/year. Combined cost £14,000 / £984 = 14.2 year combined payback — but the battery itself pays back £6,500 / (£984 − £615) ≈ 9.5-10 years against a no-solar baseline if you also exploit cheap overnight import.
Why your number may be better
If you use a smart tariff (Octopus Flux, Intelligent Go) and arbitrage overnight import, battery economics improve sharply — typically a further £150-£300 annual saving.
What drives payback up
Heavy shading from trees or neighbouring buildings can cut yield 15-30% on the wrong roof. North-facing pitches typically lose 25-30% versus south. Working away from home all day (low self-consumption) pushes solar-only payback past 12 years unless you add a battery.
Choosing premium-brand aesthetics — full-black panels, in-roof integration, Tesla Powerwall 3 over a value-engineered alternative — adds £2,000-£4,000 to system cost without changing energy output. Worth it for the look; not worth it for the maths.
What drives payback down
Being at home during the day, owning an EV, having a heat pump, or using a smart tariff with overnight cheap rates all materially shorten payback. Households combining three or more of these regularly see solar+battery payback under 7 years.
Larger system sizes typically improve £/kWp economics. The fixed costs of scaffolding, DNO paperwork and certification are the same on a 4 kWp and an 8 kWp install, so the per-kWp cost falls as you scale up — provided your roof and consumption support it.
Should you include electricity inflation in the model?
Most honest installers (us included) quote payback at current electricity prices with zero inflation built in. That keeps the figure conservative and comparable. If you add even 3% annual electricity price inflation — well below the long-run average — typical payback shortens by 12-18 months.
Be wary of installers projecting 6-10% annual electricity inflation to advertise 5-year paybacks. Those projections aren't impossible, but they aren't conservative, and they aren't how the industry's reputable players model returns.
Payback vs lifetime return
Payback is just the first milestone. A system that pays back in 9 years and runs for 25 years delivers 16 further years of essentially free generation. On a £14,000 install saving £1,000+/year, lifetime return after payback is £16,000-£25,000 in today's money — a 200-300% return on the original investment, not counting any capital uplift to your home.