PVGIS vs NASA POWER Weather Data

Solar ROI depends heavily on irradiation data. Weather source selection can change estimated generation and payback.

Why irradiation matters

PV modules convert sunlight into electricity. If annual irradiation is overestimated, production, revenue, IRR, and NPV will also be overestimated. If irradiation is underestimated, a good project may look weaker than it really is.

PVGIS

PVGIS is widely used for photovoltaic performance estimates in many regions. It can provide PV-oriented outputs and is useful when the project location is covered well by its datasets. For screening, PVGIS data is often a practical first source.

NASA POWER

NASA POWER provides global meteorological and solar data through APIs. It is useful for broad location coverage and climate-based estimates. It may not represent microclimate, local shading, rooftop conditions, or urban site effects.

POA and monthly data

PV Yield can use monthly POA values. POA means plane-of-array irradiation, which reflects the tilted surface of the modules rather than only horizontal irradiation. If you import custom POA data, check units, month order, and whether values already include orientation effects.

When POA data already includes tilt and azimuth effects, avoid applying a second orientation correction in another model. When using horizontal irradiation, make sure the calculator or design tool converts it to the module plane before estimating production.

Best practice

• Use coordinates, not only city names, for site-specific estimates.
• Compare at least two weather sources for larger projects.
• Ask installers how their production estimate handles shading and losses.
• Use conservative assumptions when the financial decision is sensitive to generation.

Uncertainty in screening models

Weather datasets describe long-term climate, not exact future output. A single year can be sunnier or cloudier than the long-term average, and rooftop shading can dominate regional weather differences. For investment decisions, use weather data together with a conservative loss case and a sensitivity check.

Weather data is an input, not a guarantee. Real production varies by year and site condition.