Ledgerly Solar Payback Calculator

What it is for

An installer's quote usually comes with a payback promise. This calculator lets you check it with your own numbers: the price, the yearly yield, how much of the power you use yourself, your electricity price, the feed-in payment and how long it is guaranteed. It runs the system year by year, including panel ageing, rising running costs, an inverter replacement and an optional loan.

It answers "when is my money back, and what do I earn on it?". It does not know your country's subsidies, tax rules or feed-in rates — you enter them. For the yield itself, start with the Rooftop Solar Yield calculator.

Worked example

A 10 kWp system for 16,000 EUR that makes 9,500 kWh in its first year. You use 28 % of it yourself and pay 0.33 EUR/kWh for grid power; the rest earns 0.0786 EUR/kWh for 20 years. Everything else stays at the defaults (0.5 % ageing a year, 150 EUR running costs, a 1,500 EUR inverter in year 15, 2 % price rise, 3 % discount rate, 25 years).

The result: payback after 12.2 years, discounted payback 16.4 years, an NPV of 5,405 EUR, a return of 5.9 % on the investment and a cost of 0.13 EUR per kWh produced. After 25 years you are 14,308 EUR ahead. In year 1 the panels save 878 EUR on the bill and earn 538 EUR feed-in; after 150 EUR running costs that is 1,265 EUR.

The 3 × 3 table shows how fragile the payback is: at 18 % self-use and a 1 % price rise it stretches to 17.5 years, at 38 % and 3 % it shrinks to 9.9 years.

Now finance the whole system with a loan of 16,000 EUR at 5 % over 15 years. The payback of the investment stays at 12.2 years, but after paying the interest you end 7,534 EUR ahead and the NPV falls to 3,279 EUR.

How to read the result

• Payback — when the savings have repaid the price minus subsidies. Discounted payback does the same with future money worth less.
• Net present value — everything in today's money at your discount rate, including loan payments. Positive means better than leaving the money where it earns the discount rate.
• Return on the investment (IRR) — the yearly return of the system itself; with a loan you also see the return on your own money.
• Cost per kWh produced (LCOE) — compare it with your electricity price and the feed-in payment.
• The chart shows your cash position year by year, starting with your own money in year 0.

Tips and common mistakes

• Self-consumption matters more than almost anything else: power you use saves the full electricity price, exported power only earns the feed-in payment.
• Take the yield from the yield calculator or PVGIS, not from a sales brochure.
• A warning about negative years usually means the inverter replacement or loan payments exceed the savings in that year — expected, not an error.
• Set Years the feed-in payment is guaranteed to what your contract says; afterwards exports earn nothing in this model.
• Use a discount rate close to what your savings would earn otherwise.

FAQ

Why does a loan not change the payback?

Payback measures the investment itself: price minus subsidies against what it saves. The loan changes when you pay, and the interest lowers your NPV and final cash, which the result shows separately.

Is income from feed-in taxed?

The model does not include taxes on feed-in income. If they apply to you, lower the feed-in payment accordingly.

What does a battery change?

Mostly the self-consumption share and the price. Run the Home Battery Simulator, then use its self-consumption here, and compare both cases as variant A and B in Ledgerly.

Why is the LCOE lower than my electricity price?

Because the panels' price is spread over 25 years of production. That gap is exactly why self-used solar power saves money.