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Guide

Methodology: how GuaranteeCheck's true-up math works

GuaranteeCheck computes the guaranteed production for the period, subtracts actual production and any carried-over surplus, and multiplies any remaining shortfall by your lease's per-kWh rate.

Last reviewed: . Facts on this page rest on GuaranteeCheck research checked on September 23, 2026.

The short answer

GuaranteeCheck's true-up math is simple arithmetic: guaranteed production for the period, minus actual production, minus any carried-over surplus. If the result is above zero, that is the shortfall, and it is multiplied by your lease's per-kWh rate to get the dollar amount.

Inputs

  • Annual guaranteed production (kWh), entered by you from your lease or PPA.
  • True-up interval in months, entered by you. It defaults to 24 months, the cycle our research found in SunPower-era leases.
  • Actual production for the period (kWh), from Enphase, or from a SolarEdge CSV export or manual entry.
  • Carried-over surplus (kWh) from the prior period, if any. It defaults to zero.
  • True-up rate ($ per kWh), entered by you from your agreement.
  • Optional: a flat annual degradation percentage and the system's age at the start of the period, used only if your lease states a degradation figure.

The formula

1. Adjust for degradation (if any)annual guarantee × (1 − degradation %) ^ system age in years
2. Scale to the periodadjusted annual guarantee × (interval in months ÷ 12)
3. Net shortfallthe larger of zero and (period guarantee − actual − carried-over surplus)
4. Amount owednet shortfall × rate, rounded to the cent

If there is no shortfall, the leftover margin becomes the surplus carried into the next period. With no degradation figure entered, step 1 changes nothing. Amounts are rounded to two decimals.

Assumptions

  • The numbers you enter match your actual agreement. GuaranteeCheck does not read or parse your lease document.
  • The guarantee is a fixed annual amount, scaled evenly across the interval.
  • Surplus offsets a later shortfall and is never paid out on its own, consistent with the mechanics our research found.

Known limits

  • Most periods show no shortfall. Our research found that most systems in most periods are owed nothing, and true-ups recur only every year or two. When a shortfall exists, our research expects the typical amount to be in the low hundreds of dollars.
  • No legal review. Contracts vary. Any adjustments, exclusions or special terms in your own agreement are not modeled unless they fit the inputs above.
  • Data quality. Results are only as good as the production data and the contract numbers entered. A wrong rate or period date gives a wrong answer, so double-check them.
  • The letter is yours. GuaranteeCheck drafts a claim letter, but you review, sign and send it yourself.

See a full hypothetical walk-through in the true-up explainer, or the same math with sample numbers on How it works.

Want the math run for you? See how the free diagnostic works, or see pricing. Common questions are answered on the FAQ.

Keep reading

Not legal advice. Not affiliated with Sunnova, SunPower, PosiGen, SunStrong, Sunrun or Renewbrook. You send the letter yourself.