Path-Q Solar

How should I compare quotes that use different panel degradation assumptions?

Answer

Compare the same equipment, starting production, annual degradation rate, forecast period, and calculation method rather than comparing headline lifetime output. Recalculate each quote using one common degradation assumption and check whether the rate is supported by the panel documentation or a contractual production guarantee.

Key points

  • A small annual-rate difference compounds over the system life.
  • Separate panel degradation from inverter replacement, shading changes, soiling, outages, and other production losses.
  • Check whether the quote models first-year degradation separately from later years.
  • Compare both annual output and the remedy if actual production falls short.

What it depends on

  • Panel technology and documented degradation information
  • The system's initial production estimate
  • The forecast horizon and loss assumptions
  • Any production guarantee and its measurement method

Risks and trade-offs

  • A lower degradation rate can make a quote appear more valuable without changing first-year output.
  • Optimistic assumptions may inflate savings or payback projections.
  • A conservative model may be more credible but show lower forecast production.

Information you may need

  • The annual production table for each quote
  • The degradation rate and source used
  • All other modeled losses and the production-guarantee terms

Your next step

What to do next

Ask installers to rerun their proposals with a common degradation rate and identical loss assumptions, then compare the resulting annual and lifetime output.

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