Path-Q Solar

How should I recalculate solar payback if production falls below forecast during the first few years?

Answer

Replace the forecast with measured production, determine whether the shortfall is temporary or structural, revise future production assumptions, and recalculate cumulative cash flow from the actual starting point.

Key points

  • Check meter data, monitoring data, outage records, curtailment, shading changes, and equipment faults before changing the model.
  • Use a revised degradation or performance assumption only after identifying the cause.
  • Include lost savings, repair costs, and any warranty recovery separately.

What it depends on

  • Actual production and consumption data
  • Original forecast assumptions and weather normalization
  • Utility bill or export records
  • Cause, duration, and recoverability of the shortfall

Risks and trade-offs

  • A short measurement period may reflect unusual weather rather than permanent underperformance.
  • Ignoring persistent underproduction overstates payback performance.
  • Corrective work may have upfront cost but improve long-term returns.

Information you may need

  • Monthly or interval production data
  • Original model and expected production by period
  • Bills, fault logs, service records, and shading or site changes

Your next step

What to do next

Reconcile measured and modeled production with the installer, document the cause, and rerun the payback model using verified actuals and revised forecasts.

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SQ02445