Path-Q Solar

How should I proceed if two installers use different weather data for the same site?

Answer

Ask both installers to rerun or clearly explain their estimates using a common weather-data source and matching assumptions before comparing results. If they cannot, compare the datasets’ period, location, resolution, and uncertainty rather than treating the annual outputs as directly equivalent.

Key points

  • Request the source, time span, geographic reference, and whether the data is measured, modeled, or blended.
  • Normalize system size, orientation, shading, equipment, loss factors, degradation, and availability assumptions.
  • Focus on the reason for the difference and consider an independent model if it affects the decision.

What it depends on

  • Weather dataset quality and relevance to the site
  • Differences in system design and loss assumptions
  • Materiality of the production difference to cost or financing

Risks and trade-offs

  • A favorable dataset can make a proposal look better without reflecting a better design.
  • A common dataset improves comparison but does not guarantee accurate future production.
  • Independent modeling adds cost and may still produce a range rather than one definitive answer.

Information you may need

  • Weather-data source and coverage for each quote
  • Complete production-model assumptions
  • Recalculated estimates using matched inputs

Your next step

What to do next

Request a side-by-side model using one agreed dataset and identical assumptions; if the difference remains material, obtain independent production modeling.

Path-Q

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SQ02048