Test the solar payback number by changing one input at a time

Define the cash-purchase baseline

Our simple model subtracts a separately confirmed non-repayable incentive from the entered installed cost, then divides by first-year energy value after an operating allowance. It excludes financing interest, discounting, future price changes, degradation and replacements. Those exclusions are important when interpreting a result that spans many years.

Use the actual installed-cost quote, including applicable taxes and required supporting work. Do not subtract a loan as though it were a grant. If annual net value is zero or negative, there is no simple recovery at those inputs; an infinite or negative payback number would not help the comparison.

Change one assumption, then explain the difference

The table uses a hypothetical $20,000 cash cost, no incentive, $100 annual operating allowance and a $0.14/kWh avoided import-energy value. Its rates and output are examples, not current market terms. The on-site-use fraction means a share of generated energy, with enough annual demand assumed for the cases shown.

Look for the assumption doing the most work

In these examples, a favourable export value changes the result materially. That tells you to read the retailer contract closely and test a less favourable value; it does not predict a future rate. A higher on-site fraction should likewise be supported by load timing rather than selected because it creates an attractive result.

Run a combined conservative case after the individual changes. Lower generation and less favourable rates can occur together. A proposal that remains understandable under that case is easier to assess than one that presents only its best result.

Use a lifecycle model for the final decision

For a major purchase, ask for a model that makes lifetime production, replacement, operating costs, financing and the time value of money explicit. Keep each assumption editable and retain the version used for the decision. Compare the solar project with other uses of the same funds and with your ownership horizon.

Simple recovery can organize questions and expose sensitivity. It cannot promise a break-even year, establish a resale premium or replace the project-specific financial and technical review.

• Capital cost and confirmed incentive recorded separately.
• Rates and yield linked to evidence.
• Individual and combined conservative cases compared.
• Excluded lifetime costs identified.
• Final decision model retained with its assumptions.