Solar panel calculator: real hourly output, savings & payback

How this solar calculator works

Most online solar calculators multiply your system size by an average number of “peak sun hours”. That shortcut hides almost everything that matters: the angle of the sun through the seasons, cloudy versus clear days, how hot the panels run in July, and how much of the power you can actually use at home. This calculator takes the long route. When you choose a location it downloads 8,760 hours of real satellite-derived weather from NASA’s POWER project (global, direct and diffuse irradiance, air temperature and wind) and simulates your system hour by hour, the way professional yield software does.

For every hour it:

1. computes the position of the sun in the sky for your coordinates;
2. splits the measured sunlight into direct beam, diffuse sky light and light reflected from the ground;
3. projects each component onto the plane of your panels using the Hay–Davies sky model (for trackers, the plane follows the sun);
4. removes reflection losses at shallow angles of incidence;
5. estimates the cell temperature from irradiance, air temperature and wind, and reduces output with the temperature coefficient of the chosen module technology;
6. applies your system losses and inverter efficiency, and clips power above the inverter’s AC rating.

The yearly sum is your production. The monthly bars and average-day curves come from the same hourly series, so they show the real seasonal shape at your site rather than a generic curve.

The core formula

Here γ is the power temperature coefficient (−0.24 %/°C for heterojunction up to −0.40 %/°C for older polycrystalline modules), U0 and U1 describe how well the modules are ventilated, and η_inv is the inverter efficiency (97% weighted).

Worked example: 6 kWp on a house in Bucharest

With the default inputs (6 kWp of TOPCon modules, 32° tilt facing due south, 14% losses and 2024 weather) the simulation gives about 7,300 kWh per year, a specific yield of roughly 1,220 kWh/kWp and a performance ratio near 77%. July produces about three times as much as December.

If the household consumes 60% of that energy directly at €0.25/kWh and exports the rest for €0.06/kWh, each kWh is worth about €0.174, so the first year saves around €1,270. With an installed cost of €6,600, maintenance of €60/year and one inverter replacement in year 13, the system pays back in a little over five years and returns more than €30,000 net over 25 years.

How to read the results

• Annual production is the first year’s AC energy at the meter. Later years decline by the degradation rate (0.3–0.5%/year for modern modules).
• Specific yield (kWh/kWp) lets you compare locations and designs regardless of size.
• Performance ratio (PR) is the share of the sunlight’s theoretical potential that reaches the grid. 75–85% is normal; a lower value points to heat, clipping or high losses.
• Irradiation on panels is the solar energy hitting one square metre of your tilted modules over the year.
• Payback, IRR and NPV compare the system against simply buying electricity. IRR above your mortgage or savings rate means the panels are the better use of the money.
• LCOE is what each kWh from your roof costs over its life. Compare it with your retail price.

Typical values

Tips to get more from your system

• Increase self-consumption. Every kWh you use yourself is worth the full retail price; exported kWh often earn a quarter of that. Run dishwashers, water heaters and EV charging at midday, or add a home battery.
• Don’t fear east–west roofs. They lose 10–20% of annual energy but produce a broader daily curve that matches household demand better.
• Oversize the array slightly. A DC/AC ratio of 1.1–1.3 is normal; the small clipping loss is outweighed by a cheaper inverter and more energy in the morning, evening and winter.
• Keep panels cool. A rack with an air gap behind the modules runs several degrees cooler than flush-mounted panels. In hot climates that is worth 2–4% more energy.
• Check the tilt. Use the solar tilt angle calculator to see how much a steeper or flatter mounting would gain or lose at your site.

Limitations

The model does not include shading from trees, chimneys or neighbouring buildings. If part of your roof is shaded, raise the system-loss percentage to match. NASA POWER weather is derived from satellites at roughly 50 km resolution, so very local effects (coastal fog, mountain valleys) are smoothed. Year-to-year weather varies by ±5–8%, which you can test with the weather year option. Financial results ignore taxes, financing and future tariff changes beyond the escalation rate you set.

Method & assumptions

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