How the model works
Each year the calculator estimates how many kWh the panels produce, splits them into self-used and exported, and prices each part.
Benefit in year n = production × self-use share × grid price × (1 + price rise)^(n − 1)
+ production × (1 − self-use share) × export rate − upkeep
The inverter replacement is subtracted in year 12. Payback is the point at which the running total of benefits passes the system cost, interpolated within the year it happens.
Worked example
A 6 kWp system producing 900 kWh per kWp makes 5,400 kWh in its first year. At 1,300 per kWp it costs 7,800. With 35 percent self-use, a grid price of 0.30, an export rate of 0.08, a 2 percent yearly price rise, 0.5 percent yearly degradation, 80 a year for upkeep and 1,000 for the inverter, the model gives:
| Result | Value |
|---|---|
| First-year net benefit | 768 |
| Payback | 9.7 years |
| 25-year profit after paying for the system | 12,839 |
| Average benefit per year | 826 |
What moves the result most
Self-use matters more than panel count. Raise self-use from 35 to 60 percent, which a household can approach by running a heat pump, charging a car or shifting laundry to midday, and payback falls from 9.7 to 7.0 years. The 25-year profit rises to 22,455. Each self-used kWh saves the full grid price, while each exported kWh earns the much lower export rate.
Grid price expectations also matter. Setting the yearly price rise to zero, a cautious assumption, stretches payback to 10.4 years and cuts the 25-year profit to 9,171. Showing both views is more honest than picking the sunniest one, so try a range before you commit.
Choosing the yield
Yield depends on location, roof angle, orientation and shading. A KernelHost explainer gives 8,500 to 10,500 kWh a year for a 10 kWp system in central Europe, which is 850 to 1,050 kWh per kWp. Southern Europe is higher and northern Europe is lower. Ask your installer for a yield estimate for your specific roof, and ask what it assumes about shading.
What the calculator leaves out
Batteries are not modelled, because they change self-use and add a significant cost. If you plan to add one, raise the self-use share to the figure your installer quotes and add the battery to the installed cost per kWp. Financing costs, roof repairs, scaffolding and the effect on your property value are also left out. Enter the installed price after any VAT relief or grant that applies to you.
Reading the answer
Payback is a simple, useful number, but it hides what happens afterwards. A system with a ten-year payback and a 25-year life spends 15 years producing profit. The tool also shows the 25-year profit so that a system with a slightly longer payback but a much larger lifetime return is not overlooked.
Frequently asked questions
How much of my output will I use myself?
A home with no battery and nobody in during the day typically uses 25 to 40 percent. Working from home, running a heat pump or charging an EV in daylight pushes the share higher. Your installer can estimate it from your hourly consumption.
Are export payments guaranteed?
No. Schemes differ by country and supplier and they change. Use the rate you are offered today, and test a lower number.
Should I include a battery?
Only if you have a quote. Enter its cost in the installed price and adjust self-use to match.
Why is there an inverter cost in year 12?
Inverters often wear out before panels do. A replacement is a realistic cost within the life of the system, and leaving it out flatters the result.