THE SHORT ANSWER
A system-loss percentage bundles several modeled effects. Save the individual inputs and avoid counting the same shade or availability loss twice.
Open the detailed inputs
PVWatts allows users to review system losses rather than only the annual result. [1] Record soiling, shading, mismatch, wiring, connections, light-induced degradation, nameplate rating, age and availability settings when used. Do not assume a default describes the property.
Ask what another tool already modeled
Some proposal platforms model shade, module behavior or inverter performance separately. Ask whether those results were also entered in the loss field. A loss counted in two places can depress the estimate; a site-specific effect omitted everywhere can make it optimistic.
Distinguish loss from consumption
Model losses reduce estimated solar energy before it reaches the stated output. They are not household use, utility export treatment, a financing fee or a battery reserve. Keep the energy model separate from bill-offset and payment calculations so each conclusion can be traced.
Run a bounded sensitivity check
Ask how annual kWh changes under a reasonable alternative loss assumption, while leaving array size and orientation fixed. The purpose is not to choose the most favorable number. It is to see which input materially drives the proposal and deserves better site evidence.
Archive the final model
Save the PVWatts result or equivalent report with location, weather dataset, array inputs and loss details. Date the file and link it to the exact proposal revision. Future actual production can be compared with context, not used to claim retroactively that an estimate was a guarantee.
Before your next conversation
- Expand the system-loss inputs.
- Check for double-counted effects.
- Keep consumption and finance math separate.
- Save the final dated model report.
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.
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 Direct to explain the loss assumptions line by line.
Book an Origin Direct review →Scope, pricing and availability are confirmed for your property. A request is not a confirmed appointment.