Should I fit the biggest system that fits?

Only if you can use the output. Beyond your self-consumption capacity, each extra panel earns the export rate — here under a third of retail — so the marginal payback on those panels is far worse than on the first ones.

How do I raise self-consumption cheaply?

Shift flexible loads into daylight hours. Timers on the dishwasher, washing machine and immersion heater cost almost nothing and can move self-use by tens of percentage points, which is worth more than several additional panels.

Solar Panel Payback Calculator – Break-even and 25-Year Return

Estimate how long a solar installation takes to pay for itself, and what it earns over its lifetime.

Annual saving$1,008

Payback period11.9 years

Net gain over 25 years$13,200

What drives solar payback

The single biggest factor is how much of your own production you actually use. Electricity you consume yourself replaces power you would have bought at the full retail price, while surplus you export is usually paid at a much lower rate. Shifting laundry, dishwashing and car charging into daylight hours therefore shortens payback more than adding panels does.

Panels lose a little output each year, typically around 0.5%, and an inverter usually needs replacing once in a 25-year life. This estimate keeps production flat and ignores that replacement, so treat the payback figure as an optimistic baseline and confirm the numbers with a quote for your roof.

Doubling your panels barely helps. Doubling your self-consumption does

Solar payback is usually pitched as a question of system size, but the arithmetic says otherwise. Electricity you use yourself is worth the retail price; electricity you export is worth a fraction of it. That gap means the share you consume on site moves the payback far more than the number of panels on the roof.

How it works

Estimates annual generation from system size and local yield.
Values self-consumed and exported electricity separately, because they are worth very different amounts.
Divides installed cost by annual value to give a payback period in years.

generation = kWp × annual yield per kWp

annual value = generation × self-use × retail price
             + generation × (1 − self-use) × export price

payback = installed cost ÷ annual value

Worked example

A 6 kWp system costing 12,000, yielding 1,000 kWh per kWp, retail 0.30 and export 0.08.

generation: 6 × 1,000 = 6,000 kWh a year
at 30% self-use: 1,800 kWh × 0.30 + 4,200 kWh × 0.08 = 876
payback: 12,000 ÷ 876 = 13.7 years
at 60% self-use: 3,600 × 0.30 + 2,400 × 0.08 = 1,272 → 9.4 years

Raising self-consumption from 30% to 60% cuts 4.3 years off the payback without adding a single panel. Doubling the array instead — 12 kWp for 22,000 at the same 30% — only improves it to 12.6 years.

Reading the result

Self-consumption is a timing problem, not a hardware one. Running the dishwasher, washing machine and hot water at midday rather than in the evening is free and moves the number immediately.
That is also the honest case for a battery: it raises self-consumption rather than generation. Whether it pays depends on whether the improvement it buys exceeds its own cost over its own lifetime, which is a separate calculation and often marginal.
Yield varies strongly with latitude, orientation and shading. Roughly 1,000 kWh per kWp suits central Europe; southern Spain approaches 1,500 and Scotland falls nearer 800, so the same system pays back on quite different timelines.
Panels degrade slowly, on the order of 0.5% a year, while electricity prices historically rise faster. The two effects partly cancel, but a payback computed at today's tariff is a conservative figure rather than an optimistic one.

Common questions

Should I fit the biggest system that fits?: Only if you can use the output. Beyond your self-consumption capacity, each extra panel earns the export rate — here under a third of retail — so the marginal payback on those panels is far worse than on the first ones.
How do I raise self-consumption cheaply?: Shift flexible loads into daylight hours. Timers on the dishwasher, washing machine and immersion heater cost almost nothing and can move self-use by tens of percentage points, which is worth more than several additional panels.