The Honest Payback Calculation for Rooftop Solar

Every solar quote contains a payback figure, and almost every one of them is optimistic. Not fraudulent — optimistic. The assumptions that produce a seven-year payback are individually defensible and collectively generous. Building the number yourself takes an afternoon and changes what you buy.

The five inputs that decide everything

A rooftop system's economics rest on five quantities. Everything else is detail.

1. Installed cost after incentives. The cash out of your pocket on day one.
2. Annual production. Kilowatt-hours the array actually delivers, not nameplate multiplied by optimism.
3. Self-consumption ratio. The share of that production you use yourself rather than exporting.
4. Import and export prices. What you avoid paying, and what you are paid for the surplus.
5. Discount rate. What the money would have earned elsewhere.

Miss any of these and the answer is wrong by a factor, not by a rounding error.

Production: start from a yield model, not from sun hours

The common shortcut — nameplate kilowatts times "peak sun hours" — ignores orientation, tilt, shading, soiling, temperature and inverter losses. Use a proper irradiance model instead. Free tools such as PVGIS or NREL's PVWatts take a location, tilt, azimuth and system loss factor and return a monthly yield profile.

Typical specific yields, in kilowatt-hours per installed kilowatt per year:

Apply a system loss factor of 12 to 16 percent for a clean, unshaded, well-ventilated array. Add more for known shading. A 6 kW system in central Europe at 1,050 kWh/kWp therefore produces about 6,300 kWh in year one — and about 5,600 kWh in year twenty-five after degradation.

Self-consumption is where most of the money is

In most markets, a kilowatt-hour you consume yourself is worth two to four times a kilowatt-hour you export. Import prices of 0.25 to 0.40 €/kWh against export rates of 0.05 to 0.12 €/kWh are common. So the value of your array depends less on how much it produces than on when it produces relative to when you use power.

A household with no battery, no electric vehicle and daytime occupancy near zero typically self-consumes 25 to 35 percent of production. Add a heat pump and the figure rises to 40 to 50 percent. Add a battery and 60 to 75 percent becomes reachable. Add an EV charged on a workday-daytime schedule and it climbs further.

The financial consequence is stark. Take the 6,300 kWh system above, an import price of 0.32 €/kWh and an export price of 0.08 €/kWh:

• At 30 percent self-consumption: (1,890 × 0.32) + (4,410 × 0.08) = €958 per year
• At 60 percent self-consumption: (3,780 × 0.32) + (2,520 × 0.08) = €1,411 per year

Same hardware, same sun, 47 percent more value. This is why load shifting — running the dishwasher, the washing machine, the water heater and the car charger during the solar window — is the highest-return action available to a solar owner and costs nothing.

Building the cash flow

Simple payback is installed cost divided by first-year saving. It is easy, quoted everywhere, and wrong in two directions at once: it ignores that electricity prices rise, and it ignores that money has a time value.

A defensible model runs year by year:

• Start with year-one saving.
• Escalate the electricity price by a modest real rate. Two percent above inflation is defensible; five is wishful.
• Degrade production by 0.5 percent per year.
• Subtract maintenance. Budget €50 to €100 per year for cleaning and inspection.
• Subtract an inverter replacement, typically €900 to €1,600, somewhere between years twelve and fifteen.
• Discount each year's net saving back to present value.

With €8,500 installed cost, €1,100 first-year saving at 45 percent self-consumption, 2 percent real price escalation, 0.5 percent degradation, €70 annual maintenance, a €1,200 inverter in year thirteen and a 4 percent discount rate, the discounted payback lands around year ten and the twenty-five-year net present value is roughly €9,000. That is a real return in the region of 8 to 9 percent, tax-free, low-risk — considerably better than most people expect and considerably worse than the seven years on the quote.

What the quote usually gets wrong

Degradation omitted. Costs you roughly 6 percent of lifetime output.

No inverter replacement. A string inverter's design life is typically twelve to fifteen years, and microinverter and optimiser warranties do not cover labour.

Export price assumed constant. Export tariffs have fallen in almost every mature market as penetration has risen. Assuming today's rate for twenty-five years is a bet.

Self-consumption assumed high without a battery. If the quote claims 60 percent self-consumption for a system with no storage and a household that is out during the day, ask where the number came from.

Discount rate of zero. Implicitly assumes your capital had no alternative use.

Batteries change the sums, usually modestly

A battery raises self-consumption, and in markets with time-of-use tariffs it can also arbitrage cheap night rates. It also costs €600 to €1,000 per usable kilowatt-hour installed and carries its own degradation and its own warranty horizon, typically ten years or a cycle count.

The test is simple: value of additional self-consumed energy per year, divided by installed battery cost. A 10 kWh battery that shifts 2,400 kWh per year from export to self-consumption at a 0.24 €/kWh spread earns €576 per year against perhaps €7,500 installed — a thirteen-year payback on a ten-year warranty. It becomes attractive where the import-export spread is wide, where a time-of-use tariff adds an arbitrage margin, or where backup power has value to you that is not purely financial.

Financing changes the question

Paying cash means comparing the solar return against your next-best investment. Borrowing means comparing the solar return against the loan rate. If the system yields 8 percent and the loan costs 5 percent, the spread is positive from day one and the system is cash-flow positive from the first month — you simply own less of it at the start.

Leases and power purchase agreements are different animals. They typically remove the upfront cost in exchange for most of the return, and the contract terms — escalators, buyout clauses, obligations on sale of the house — deserve a careful read. A 3 percent annual escalator in a PPA can outrun electricity price growth and leave you paying above market by year fifteen.

Frequently asked questions

Does solar increase property value? Studies in several markets show a premium, generally for owned systems rather than leased ones. Leased systems can complicate a sale because the buyer must accept the contract.

What about maintenance? Little. An annual visual inspection, cleaning in dusty or pollen-heavy locations, and monitoring that actually gets looked at. The most common maintenance failure is an inverter that has been offline for months without anyone noticing.

Should I oversize the array relative to the inverter? Usually yes. A DC-to-AC ratio of 1.15 to 1.3 raises annual yield because the array rarely reaches nameplate. Clipping losses at that ratio are typically under 2 percent.

Is it better to wait for cheaper panels? Module prices fall slowly now; installation labour and scaffolding do not fall at all. Meanwhile every year of waiting forfeits a year of savings. The arithmetic almost always favours acting.

The number that actually matters

Payback is a comforting metric and a poor one. It ignores everything that happens after the payback date, which for a twenty-five-year asset is most of the value. Net present value and internal rate of return answer the real question: is this a better use of capital than the alternatives? For a well-designed system on a suitable roof in a market with retail electricity above 0.20 €/kWh, the answer is usually yes — by a comfortable margin, and by less of one than the brochure claims.