Solar payback period, NPV, and IRR: what the numbers really mean
Published 21 June 2026
Three numbers describe a solar investment. Payback is the time until cumulative savings repay the upfront cost. NPV is the total gain over the system's life in today's money, after discounting. IRR is that gain expressed as an annual percentage return. Read all three together, because each hides what the others reveal.
A solar salesperson tells you the panels pay for themselves in eight years. You nod, because eight sounds better than ten. Then you sign, and you never find out whether eight years was a good deal or a mediocre one. Payback alone cannot tell you that.
Three numbers describe a solar investment: payback period, net present value (NPV), and internal rate of return (IRR). Each answers a different question. Payback tells you when your money comes home. NPV tells you how much you gain in today's money over the full life of the system. IRR turns that gain into a percentage you can compare to any other investment. You need all three because each one hides something the others reveal.
Payback period: when does your money come home?
Payback measures the time until your cumulative savings equal what you spent. Pay (for example) the equivalent of €8,000 for a system, and it saves you roughly 12–15% of your annual electricity bill. After eight years you have recovered your outlay. That is your payback period.
You can feel this number in your gut, which is why everyone leads with it. It tells you how long your money sits at risk before it comes home.
It also lies by omission. Payback stops counting the day you break even. A system that lasts 25 years keeps earning for 17 more years after an eight-year payback, and payback ignores every bit of that. Two systems can both pay back in eight years while one earns double over its life. Payback cannot see the difference.
There is something else it ignores. A unit of money you save in year twelve is worth less than the same amount saved today, because today's money can sit in an account and grow. Payback treats all savings as equal regardless of when they arrive. They are not, and the next two numbers fix that.
NPV: how much do you actually gain, in today's money?
Net present value answers the question payback dodges. Across the system's whole life, how much are you ahead, counted in money you can compare to the cash in your account right now?
The trick is discounting. Take every future year of savings and shrink it to its value today. If money is worth 5% a year to you, then a sum saved in twelve years is worth only about 56% of its face value today, because that smaller amount invested at 5% would grow to match it in that time. NPV does this to every year of savings, adds them up, and subtracts what you paid upfront.
Worked example
| Input | Value |
|---|---|
| System cost | €8,000 (use the calculator for your country's equivalent) |
| Year-one savings | €1,000 (roughly 12–15% of a typical household bill) |
| Panel degradation | 0.5% per year |
| Electricity price inflation | 3% per year (illustrative midpoint; Eurostat data show EU household prices grew at roughly 2–4% per year over 2015–2025 (Eurostat nrg_pc_204, 2026), including a sharp 2021–2023 spike and a subsequent moderation) |
| Discount rate | 5% |
| System lifetime | 25 years |
The 0.5% annual degradation figure is consistent with NREL's field-degradation research, which finds a median rate of around 0.5% per year for crystalline-silicon panels (NREL Compendium of PV Degradation Rates). Most manufacturers back this with a 25-year performance warranty guaranteeing at least 80% of original output at end of life.
Working through year by year, shrinking each year's savings for panel ageing, then growing it back for rising electricity prices, then discounting to today:
- Sum of discounted savings over 25 years: ~€18,100
- Subtract the €8,000 upfront cost
- NPV = ~€10,100
That €10,100 is the prize. It says: after accounting for the cost, the timing, and the slow fade of the panels, you end up about €10,100 richer in today's money than if you had never installed anything. A positive NPV means the investment beats leaving your cash where it was. A negative NPV means it does not, regardless of how the payback looks.
NPV depends on the discount rate you choose, and that choice is yours. If you would otherwise overpay a mortgage at 5%, use 5%. If your cash earns 2% in a savings account, use 2%, and the NPV climbs higher. We show the rate we used and let you change it, because a single hidden assumption can swing the whole answer.
IRR: what annual return does the investment earn?
NPV gives you a lump sum. The internal rate of return turns the same cash flows into a percentage, so you can hold solar next to everything else competing for your money.
IRR is the discount rate at which NPV would hit exactly zero. Put another way, it is the annual return the investment earns on the money you have tied up in it. For the €8,000 system above, the IRR works out to ~14%.
Now you have a number that travels. A savings account pays 3%. Overpaying the mortgage saves you 5%. The panels return 14%. Stated that way, the comparison stops being about solar and starts being about where your money works hardest.
IRR has a blind spot of its own. A 25% return on a small system can move less money into your pocket than a 14% return on a large one, because percentages say nothing about scale. This is why you keep NPV beside it. NPV tells you the size of the win; IRR tells you the quality of it. Read them together.
Summary: three numbers, three questions
| Metric | Question answered | What it misses |
|---|---|---|
| Payback period | When do I break even? | All earnings after break-even; time value of money |
| NPV | How much do I gain, in today's money? | Depends on your chosen discount rate |
| IRR | What annual return does solar earn? | Scale: a high % on a small system may mean less cash than a lower % on a large one |
Why we show all three before you talk to anyone
Most comparison sites make you hand over your phone number before they reveal a single figure. Then an installer calls you for a fortnight. We took the opposite stance. We run the real economics first, on your actual roof, using free government climate data: PVGIS for Europe (European Commission Joint Research Centre), and NASA/NREL data elsewhere. You see the payback, the NPV, the IRR, and the assumptions behind them before any form appears.
The method works the same whether your roof sits in Lyon, Lisbon, or Limerick. We pull the local irradiance and the average tariff for your country, so the answer reflects where you live rather than a generic default that means nothing to you. The IEA Renewables 2025 report notes that cost-competitiveness has improved steadily and distributed solar continues to grow as households seek to cut bills. Paybacks vary substantially by location: in practice, high-sun, high-tariff markets (parts of southern Europe, Australia, and high-irradiance developing economies) commonly achieve payback well inside ten years, while lower-irradiance or lower-tariff markets typically see paybacks in the ten-to-fourteen-year range.
If you want to understand where the savings figure comes from, read how we size a solar system and how to read a solar quote. Once you have those, the three numbers above do the rest.
Look at payback for the gut feel. Look at NPV for the size of the gain. Look at IRR to compare it against everything else competing for your money. One number was never going to be enough.
Frequently asked questions
What is a good solar payback period? In high-irradiance, high-tariff European markets (Germany, Netherlands, Belgium) a typical residential system pays back in six to ten years. Lower-sun or lower-tariff locations often see ten to fourteen years. The IEA notes that cost-competitiveness has improved steadily; paybacks in 2024–25 are shorter than in previous years for most markets.
What discount rate should I use for a solar NPV calculation? Use the rate that reflects your best alternative use of the money. If you carry a mortgage at 4%, use 4%: solar that beats 4% is worth more than overpaying the mortgage. If your savings account earns 2.5%, use 2.5%. A higher discount rate lowers NPV; a lower rate raises it.
Is a 14% IRR for solar realistic? For a well-sited European system installed in 2025–2026, with electricity at around €0.27–0.37/kWh, reasonable panel output, 3% annual tariff growth, and a competitive install cost, yes, 14% is achievable. Systems in lower-tariff or lower-irradiance locations will show lower IRRs.
Does solar NPV stay positive if electricity prices fall? It can still be positive but will be lower. Our calculator lets you set your own inflation assumption. At 0% electricity price growth the NPV on the worked example above drops to around €5,400; at 5% growth it rises to around €14,400. The payback period barely changes; NPV is what captures the difference.
How does panel degradation affect the long-run numbers? NREL's research puts median modern-panel degradation at around 0.5% per year. At 0.5% annual loss, a panel producing 1,000 kWh in year one produces about 887 kWh in year 25, roughly an 11% cumulative fall. This is already baked into the worked example above. Higher degradation (say, 1% per year) would reduce the 25-year NPV by roughly €800–1,200 at the same assumptions.
Estimate only, not a quote. Figures use local climate data and average tariffs; your installer's quote will be exact.
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