Path-Q Solar

Should a solar production estimate use long-term historical weather data, recent local weather data, or both?

Answer

A robust estimate should generally use long-term historical data as the primary basis and recent local data as a reasonableness check, rather than relying only on either one. Long-term data better represents normal variation, while recent data can reveal changes in local conditions or unusual weather.

Key points

  • Long-term data helps avoid overreacting to an unusually sunny or cloudy recent period.
  • Recent local measurements can improve relevance when the site differs from a broader regional dataset.
  • The model should disclose the weather source, time span, location, interval, and treatment of missing data.
  • Production estimates should include assumptions for shading, orientation, tilt, temperature, soiling, equipment losses, and degradation.
  • Comparing multiple modeling approaches can reveal whether the forecast is unusually optimistic.

What it depends on

  • The quality and location of the weather dataset
  • How representative the weather station or satellite data is of the property
  • The array's orientation, tilt, shading, and equipment assumptions
  • Whether the estimate is intended for long-term planning or short-term operational forecasting

Risks and trade-offs

  • Recent weather may be more locally relevant but can be statistically unrepresentative.
  • Long-term regional data may smooth local effects or miss microclimates.
  • Different providers may produce different estimates even with similar equipment because of data and loss assumptions.

Information you may need

  • The weather source and historical period used
  • The distance and elevation difference between the data source and property
  • Monthly production estimates and modeled loss assumptions
  • Any recent local weather or monitored solar data available

Your next step

What to do next

Ask the installer for the modeling assumptions and request a sensitivity comparison using long-term data, recent local data, and a conservative production case.

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SQ00413