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Ledgerly Rooftop Solar Yield

How much power would solar panels on my roof make? Month by month, offline or with PVGIS.

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PVGIS convention: 0 = south, −90 = east, 90 = west, 180 = north.

Cables, inverter, dirt and mismatch; 14 % is the PVGIS default.

Same order as PVGIS: the first value is north, then clockwise, e.g. 8 values for N, NE, E, SE, S, SW, W, NW.

Like Hearth’s Sun Windows: compass bearing from north (°), angular width (°), height above the panels (m) and distance (m), e.g. 160, 30, 8, 12.

Only shifts the hourly profile from solar time to your clock; daylight saving time is ignored.

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More about this tool

How it works

Estimates the yield of a rooftop PV system month by month from its place, tilt, orientation, size and shading: offline from DWD climate normals with a documented solar model, or with PVGIS satellite data from the EU Joint Research Centre — always with the offline estimate beside it. Includes an average day per month for battery planning. Part of Ledgerly’s Solar & Heat.

  • Offline: daily horizontal irradiation per month from sunshine hours (Angstrom-Prescott, FAO-56 coefficients a = 0.25, b = 0.50) of the nearest DWD climate station (1991–2020, within 50 km), or from your own monthly sunshine hours or irradiation.
  • Beam and diffuse parts from the Erbs monthly correlation; spread over a typical day with the Collares-Pereira & Rabl and Liu & Jordan hour fractions; transposed to the panel plane with an isotropic sky and 20 % ground reflection (Duffie & Beckman, chapters 1–2), in 6-minute steps.
  • Shading: direct light is blocked whenever the sun is below your horizon (heights from north clockwise, as in PVGIS) or behind an obstacle given like in Hearth’s Sun Windows (bearing, width, height, distance); the horizon also removes its share of the sky light.
  • Power: cell temperature from NOCT 45 °C, −0.4 % per °C above 25 °C, reflection losses (ASHRAE, b0 = 0.05) and your system losses.
  • PVGIS: the PVcalc service of the EU Joint Research Centre (SARAH3 satellite and ERA5 data) is called from this server, never from your browser, for the place rounded to 0.01° (about 1 km); answers are cached for 30 days. The offline estimate is always computed beside it, and the average day per month is scaled to PVGIS’s monthly energy.

An estimate, not a yield guarantee or installer advice: real years vary by about ±10 %, and the roof, wiring and grid connection must be planned by a qualified installer following local codes (e.g. VDE, NEC).

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How to use the Rooftop Solar Yield calculator

Estimate what rooftop solar panels would produce each month — offline from climate normals or with PVGIS satellite data — and read the result.

What it is for

Before you ask installers for quotes it helps to know roughly what your roof can produce. This calculator estimates the yearly and monthly output of a PV system from its place, size, tilt, orientation and shading. It also gives an average day per month, which the Home Battery Simulator uses.

There are two data sources. The offline estimate uses the long-term sunshine hours and temperatures of the nearest German DWD weather station and a textbook solar model, and shows every step. PVGIS asks the EU Joint Research Centre's satellite-based service for your place; the offline estimate is always shown beside it. It does not plan wiring, check your roof's load capacity or size an inverter.

Worked example

A 6 kWp system on an east-west roof near Hamburg: latitude 53.55, longitude 9.99, Peak power (kWp) 6, Tilt 20°, Orientation −90 and Panel layout set to two halves facing opposite ways.

The offline estimate uses the DWD station Ahrensburg-Wulfsdorf, 18.6 km away, and returns about 5,131 kWh a year (855 kWh per kWp). June brings 772 kWh, December only 79 kWh. The same 6 kWp facing south at 35° would make 6,174 kWh (1,029 kWh per kWp) — the east-west roof gives about 17 % less in total, but spreads it more evenly over the day.

Switching Data source to PVGIS gives 4,837 kWh a year (PVGIS-SARAH3, 2005–2023) next to the offline 5,131 kWh. A difference of around 5 % is typical; PVGIS uses measured satellite data for your exact place and is usually closer.

Add a tree in the east with Obstacles 100, 40, 10, 12 (bearing 100°, 40° wide, 10 m above the panels, 12 m away) and the yield drops to 4,753 kWh, with Lost to horizon and obstacles at 7.6 %.

How to read the result

  • Yield per year and Yield per kWp — the headline numbers. Central Europe typically lands between 800 and 1,100 kWh per kWp.
  • Sunlight on the panels per year — irradiation on the tilted plane in kWh/m², before losses.
  • Performance ratio — how much of that sunlight ends up as electricity after heat, reflection and system losses.
  • The bar chart and the Month by month table show sunlight on the ground, its diffuse share, sunlight on the panels, the mean temperature and the energy per month.

Tips and common mistakes

  • Orientation uses the PVGIS convention: 0 = south, −90 = east, 90 = west, 180 = north.
  • Leave System losses at 14 % unless you know better; it covers cables, inverter, dirt and mismatch.
  • Measure obstacles from the middle of the panel area; height means above the panels, not above the ground.
  • Outside Germany there is no station within 50 km, so the offline figure is only a rough guess. Use PVGIS or enter monthly sunshine hours or irradiation yourself.
  • The time zone only shifts the hourly profile; it does not change the yearly total.

FAQ

Why do PVGIS and the offline estimate differ?

They use different data: decades of station sunshine hours versus satellite irradiation for your exact square kilometre. Winter months differ most, because sunshine hours turn into irradiation less reliably when the sun is low.

What is sent to PVGIS?

Only the place rounded to 0.01° (about 1 km), the peak power, losses, tilt, orientation and your horizon. The request comes from the usefulweb server, never from your browser, and answers are cached for 30 days.

Does it include snow, dirt or panel ageing?

Not separately. System losses cover typical soiling; ageing is handled in the Solar Payback Calculator.

Can I use it for balcony panels?

Yes. Set the tilt to 90° for panels on a railing and add the building across the street as an obstacle.

Guide last updated:

For scripts and AI agents

  • JSON API: POST https://ledgerlysolaryield.usefulweb.app/api/v1/calculate (same calculation as this page)
  • Interface description: OpenAPI 3.1 · AI guide: llms.txt
  • MCP server (Streamable HTTP): https://ledgerlysolaryield.usefulweb.app/mcp · tool name ledgerly_solar_yield
Example request
curl -s -X POST https://ledgerlysolaryield.usefulweb.app/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{"latitude":52.52,"longitude":13.405,"peakPowerKw":8,"tilt":35,"azimuth":0}'
Feedback · ideas & bug reports welcome