THE SHORT ANSWER
The array’s DC capacity and the inverter’s AC rating must agree with the model inputs. A production report can look precise while representing different equipment from the written quote. The SAM/PVWatts documentation provides a capacity-to-ratio calculation to check this. [1]
Work backward from the modeled ratio
In a hypothetical report with 12 kW DC and a DC-to-AC ratio of 1.2, the implied AC rating is 12 divided by 1.2, or 10 kW. Compare that with the actual inverter schedule. The calculation does not verify equipment compatibility or utility approval; it identifies a question if the model and the proposal do not describe the same capacity.
Do not mistake a ratio for a loss percentage
A ratio of 1.2 does not mean twenty percent of annual solar production is necessarily lost. It compares two nameplate capacities. The timing of output and the model’s treatment of inverter limits determine the energy result. The official documentation explains that PVWatts limits modeled inverter output to its AC rating. [1] Ask for the resulting annual and monthly energy comparison, not an invented loss derived directly from the ratio.
Compare designs with everything else held steady
If the provider proposes a different inverter size, request a second model using the same array layout, location and other assumptions. Record what changed and how much modeled output changed. Then compare the added equipment and installation cost. A higher capacity is not automatically worth its incremental price, and a lower capacity is not automatically an error. The specific design and purpose control that discussion.
Keep interconnection rating out of this shortcut
A utility may use its own definition of gross system rating for an application. Do not substitute this model ratio calculation for that requirement. Put the modeling capacity and the utility application classification on separate lines. This avoids using an inverter change to claim a different utility tier without checking the actual published method.
Ask for a matched final package
Before approving a proposal, compare the module schedule, inverter schedule and production report revision. If any item changes, ask whether the model was updated. For an Origin Direct conversation, bring the page that lists capacity and the report’s assumptions; you do not need to reproduce a complete engineering model yourself. The homeowner’s check is consistency, while the professional remains responsible for a suitable design.
Before your next conversation
- Calculate implied AC rating from DC capacity and ratio.
- Do not turn the ratio into an annual loss percentage.
- Match the model revision to the equipment schedule.
Primary sources
- National Laboratory of the Rockies: PVWatts System Design
Primary reference checked September 9, 2026. Applies only to the named program or equipment; confirm current terms for your property.
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.
Make the decision specific to your home.
Ask Origin to explain the inverter sizing assumption in your modeled production.
Book an Origin Direct review →Scope, pricing and availability are confirmed for your property. A consultation does not establish utility or incentive approval.