Solar Panel Payback Calculator
Work out what a solar system generates, what it saves once export and self-consumption are separated, and how long it takes to pay for itself.
How to use this calculator
- 1Enter the system size and installed cost, after any grant or tax credit.
- 2Pick your region, or enter a yield figure from a quote or from PVWatts.
- 3Set your import and export prices, and be realistic about self-consumption.
- 4Check how much the payback moves when you change the price-rise assumption.
How the calculation works
generation = kWp × yield × (1 − degradation)^(year−1) saving = self-used × import price + exported × export price payback = the year cumulative saving passes the cost- kWp
- Peak system capacity, the headline size of the array
- yield
- Annual kWh produced per kWp installed — 750 to 1,700 depending on location
- self-consumption
- The share used as it is generated, which saves the full import price
- blended rate
- The real average value of a generated unit, once export is counted separately
Solar only saves the retail electricity price on units consumed at the moment they are generated. Everything else is exported at a rate typically half the import price or less, so the effective value of a kWh is well below the headline tariff.
This is why self-consumption matters more than system size. Doubling the array on a house that exports most of its output roughly doubles the cheap export income, not the expensive bill saving.
Panels degrade slowly — around 0.5% a year is typical, and most warranties guarantee 80–85% of rated output at 25 years. Over a long model this partly cancels against rising prices.
Payback is highly sensitive to the assumed electricity price path, which nobody can forecast over 25 years. Treat any single payback figure as one scenario rather than a prediction.
Worked example
A 4 kWp UK system at £7,000
- 1.Generation: 4 kWp × 950 kWh/kWp = 3,800 kWh in year one.
- 2.Self-used at 35%: 1,330 kWh, saving 1,330 × £0.27 = £359.10.
- 3.Exported: 2,470 kWh at £0.15 = £370.50.
- 4.Year-one saving: £729.60, so the blended value is about £0.19 per kWh — well under the £0.27 tariff.
- 5.On flat prices that is a 9.6-year payback; with 3% annual price rises it comes forward to about 8.7 years.
- 6.Note that the exported two-thirds contributes barely half the saving.
Result: 8.7 years to pay back
The same system with a battery raising self-consumption
- 1.Same 3,800 kWh, but 75% is now self-used: 2,850 kWh at £0.27 = £769.50.
- 2.Only 950 kWh is exported, at £0.15 = £142.50.
- 3.Year-one saving rises to £912, and the blended rate to £0.24 per kWh.
- 4.But the battery pushed the cost from £7,000 to £11,000.
- 5.Payback lengthens rather than shortens, because the extra £4,000 buys only £182 a year of additional saving.
- 6.Batteries improve self-consumption and rarely improve payback at current prices — that is the honest arithmetic.
Result: Better self-consumption, worse payback
Export is where the economics actually live
The intuitive model of solar is that every unit generated is a unit you do not buy. That is only true for electricity used at the instant it is produced. Solar peaks around midday; household demand peaks in the morning and evening. The mismatch is the whole problem.
A typical household with nobody home during the day self-consumes 25–40% of what its panels make. The rest is exported, at a rate usually well below the import price — often half or less. So the real value of a generated kWh is a blend, and it sits much closer to the export rate than most quotes imply.
This inverts some intuitions. A larger array on a low-consumption house adds mostly cheap export income rather than expensive bill savings, so payback per pound spent gets worse, not better. Sizing the system to the daytime load usually beats filling the roof.
Why batteries improve self-consumption but rarely payback
A home battery stores midday surplus for the evening peak, which can push self-consumption from 35% to 70% or more. The saving genuinely improves — but the arithmetic frequently does not.
On the figures in the second worked example, adding a battery raises the annual saving by around £180 while adding £4,000 to the cost. That is a simple payback on the battery alone of more than twenty years, against a warranty that typically runs ten.
Batteries can still make sense for other reasons: backup during outages, arbitraging a time-of-use tariff by charging on cheap overnight rates, or simply wanting to use your own generation. Those are real benefits. Straightforward payback on the extra capital, at current battery prices and export rates, is usually not one of them.
The number nobody can honestly forecast
Every solar payback figure rests on an assumed electricity price path, and 25 years is far beyond the horizon of any credible forecast. UK domestic electricity roughly doubled between 2021 and 2023 and then partly fell back; anyone who had modelled a smooth 3% would have been wrong in both directions within two years.
The sensitivity is large. On the worked example, assuming flat prices gives a 9.6-year payback and 3% growth gives 8.2. At 6% it drops nearer 7. The system does not change at all — only a number nobody knows.
The practical response is to treat payback as a range rather than a figure, and to note that solar is unusual as an investment in that its return is denominated in avoided electricity rather than cash. If prices rise sharply it performs well precisely when you most need it to, which is a form of hedge that a straight payback number does not capture.
What this assumes, and where it stops
Assumptions
- Regional yield figures are indicative averages for a well-oriented, unshaded array.
- Self-consumption stays constant over the modelled period.
- Panel output degrades at the stated rate compounding annually.
- Import and export prices both rise at the stated inflation rate.
- The installed cost is net of any grant, subsidy or tax credit.
Limitations
- Yield depends heavily on roof pitch, orientation and shading, which can move output 30% either way from the regional average. A site-specific figure from a quote or PVWatts is far better.
- Assumes a constant self-consumption share. In reality it varies by season, weather and occupancy.
- Does not model inverter replacement, typically needed once in a 25-year life at meaningful cost.
- Ignores maintenance, cleaning, insurance and any effect on property value.
- Payback is highly sensitive to the assumed electricity price path, which cannot be forecast over decades. Treat it as one scenario.
- Not financial advice, and does not model financing costs where the system is bought on credit.
Common questions
How long do solar panels take to pay for themselves?
Typically 8 to 12 years in the UK and 6 to 10 in sunnier parts of the US, but it depends far more on self-consumption than on system size. A household using 35% of its generation gets a blended value well below its import tariff, because the exported majority earns a much lower rate.
Why does self-consumption matter more than system size?
Because only electricity used as it is generated saves the full retail price. Everything exported earns the export rate, often half as much or less. Adding panels to a house that already exports most of its output mostly adds cheap export income, so payback per pound spent gets worse rather than better.
Does a battery make solar pay back faster?
Usually not. A battery can raise self-consumption from around 35% to 70%, which genuinely increases savings — but the extra capital is often several thousand against a couple of hundred a year of additional benefit. That is a longer payback than most battery warranties. Batteries can still be worth it for backup or time-of-use tariffs.
How much electricity does a solar system generate?
Roughly 750 to 1,000 kWh per kWp per year in the UK depending on region, and 1,050 to 1,700 in the US. A typical 4 kWp UK system produces around 3,400 to 4,000 kWh annually. Roof pitch, orientation and shading can shift that by 30% either way.
Sources
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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