THE SHORT ANSWER
A first-year estimate and a multi-year projection answer different questions. Require the starting value, annual change assumption and warranty source to be shown separately.
Identify the starting year
Ask whether the advertised annual kWh represents the first full operating year, a typical meteorological year, or another period. PVWatts produces an estimate from selected system and weather inputs. [1] Keep that result separate from later spreadsheet projections.
Locate the annual change
If a proposal forecasts decades, find the degradation or production-change assumption and whether it compounds. Ask who supplied the value. Do not infer a universal rate from a salesperson’s chart or apply a module warranty number automatically to whole-system energy.
Read the actual warranty
FTC guidance advises consumers to understand warranty and contract terms. [2] Obtain the manufacturer document for the exact module and distinguish a product defect warranty from a performance-related provision, labor coverage and the installer’s separate obligations.
Keep weather variability visible
One actual year can be above or below a modeled value because weather and operating conditions differ. A long-term chart should not appear as a staircase of guaranteed utility savings. Ask which rows are model outputs, assumptions or contractual promises, if any.
Preserve a reproducible projection
Save the first-year model, annual-change formula, equipment warranty and proposal version. If equipment changes, rebuild the projection rather than swapping a logo. The goal is a transparent planning record, not a promise that every calendar year will match a spreadsheet.
Before your next conversation
- Define the first-year estimate.
- Find the annual-change formula.
- Read the exact equipment warranty.
- Separate projections from contractual promises.
Primary sources
- National Renewable Energy Laboratory: PVWatts Calculator
Primary source reviewed September 10, 2026. Utility rules, equipment availability, project design and contract terms remain specific to the property and proposal.
- Federal Trade Commission: Solar Power for Your Home
Primary source reviewed September 10, 2026. Utility rules, equipment availability, project design and contract terms remain specific to the property and proposal.
Planning examples and questions are Origin’s editorial guidance, not a property-specific diagnosis, engineering design, tax determination or promise of savings. Manufacturer and utility references do not imply an affiliation with Origin.
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