THE SHORT ANSWER
Annual consumption, modeled production and bill offset are related but not identical. Require the proposal to show the source and calculation for each.
Verify consumption first
Annual household consumption should come from a dated utility kWh history with adjustments clearly labeled. DOE notes that electricity use and utility treatment affect solar outcomes. [1] A future-load scenario should appear separately from observed usage.
Inspect production inputs
Modeled annual solar production depends on system size, orientation, tilt, losses and weather data. PVWatts exposes these inputs for an estimate. [2] Ask for the model report, not only a percentage of household use.
Define the percentage denominator
A proposal may divide production by consumption and call the result offset. Confirm the period and scenario used in both numerator and denominator. A value over 100 percent does not by itself explain export value or whether the utility permits the proposed size.
Translate energy into billing cautiously
The home can consume solar directly and exchange energy with the grid at other times. Rates, fixed charges, export compensation and time patterns affect bills. Do not convert an energy-offset percentage directly into a promised dollar reduction without the tariff assumptions.
Keep a three-line audit
Record annual consumption, modeled production and the stated bill result on separate lines with sources and version dates. If one changes, rebuild dependent calculations. This prevents an updated layout from retaining an old offset label and makes competing proposals easier to compare.
Before your next conversation
- Source the annual consumption figure.
- Save the production model.
- Define the offset equation.
- Document the billing assumptions separately.
Primary sources
- U.S. Department of Energy: Homeowner’s Guide to Going Solar
Primary source reviewed September 10, 2026. Utility rules, equipment availability, project design and contract terms remain specific to the property and proposal.
- 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.
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