THE SHORT ANSWER
PVWatts system losses are model inputs for effects not otherwise calculated in the selected setup. Save the category values, identify effects handled elsewhere and check the combined treatment before comparing annual kWh. The current PVWatts interface exposes system-loss categories alongside the other design inputs and cautions that results contain assumptions and uncertainty. [1]
Expand the categories instead of copying one total
Record the PVWatts version and every visible loss input used by the proposal. The calculator’s categories include soiling, shading, snow, mismatch, wiring, connections, light-induced degradation, nameplate rating, age and availability. [1] A combined loss value is not a description of this home, and changing one category can change the total. Ask whether the proposal used defaults, site-specific entries or an external model. Preserve the individual values and the reported combined result so another reviewer can reproduce the exact run.
Mark what PVWatts already models elsewhere
Keep inverter efficiency and DC-to-AC ratio in their own rows because the current calculator exposes them as separate system inputs. [1] Also record module type, array type, tilt, azimuth, weather station or resource and albedo or bifacial selections when used. Do not add an extra “inverter loss” merely because the proposal has an inverter, and do not treat every difference between DC nameplate and AC energy as part of the system-loss percentage. The model boundary must be stated before a homeowner can identify duplication.
Audit imported shade, soiling and availability
Ask whether a design platform already calculated shade by roof plane, whether a site study supplied a shade factor, whether soiling came from a generic assumption and what “availability” means in the proposal. Trace each effect into one model field. A measured shade result that reduces production in one tool and is then added again as a PVWatts shading loss can depress the estimate twice; a shade label that never reaches the energy model can make the estimate optimistic. The same logic applies to other imported adjustments.
Keep model losses separate from household and billing math
System losses affect modeled solar energy. They are not household consumption, a utility export rate, fixed customer charge, financing fee, tax outcome or battery reserve. DOE says potential savings depend on consumption, system size, ownership choice, production, roof direction, sunlight, utility rates and export compensation. [2] That list crosses several worksheets; it is not one PVWatts percentage. Require separate records for production, consumption, utility treatment and payment so a favorable assumption in one cannot hide an unfavorable one elsewhere.
Run a controlled sensitivity check
Duplicate the final model and change only the uncertain loss input being tested. Keep capacity, orientation, weather resource and other fields fixed, then record the annual kWh difference. This is a hypothetical comparison, not a request to choose whichever value makes the proposal look best. Its purpose is to identify which assumption materially drives the result and whether better evidence is worth obtaining. If several inputs change between runs, the homeowner cannot attribute the output difference to the loss under review.
Archive the exact run that supports the proposal
Save the calculator or model report, input list, location or station, annual and monthly results, file date and matching proposal version. If the site survey changes shade, module count, orientation or equipment, rerun the model and mark the older output superseded. PVWatts estimates potential production; it does not guarantee a specific weather year, operating condition or bill. [1] A complete archive supports a fair quote review now and a more informed production conversation later without converting an estimate into a warranty claim.
Before your next conversation
- Save the PVWatts version and every loss category.
- Identify effects modeled outside the loss stack.
- Check shade, soiling and availability for double counting.
- Run one-variable sensitivity comparisons.
- Archive the exact inputs and output with the proposal revision.
Primary sources
- National Renewable Energy Laboratory: PVWatts Calculator
Primary source reviewed October 10, 2026. Utility rules, equipment availability, project design and contract terms remain specific to the property and proposal.
- U.S. Department of Energy: Homeowner’s Guide to Solar
Primary source reviewed October 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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