THE SHORT ANSWER
Kilowatts describe rated power at defined conditions; kilowatt-hours describe energy across time. Recalculate the proposed kWdc from the equipment schedule, then require the annual kWh estimate to carry its location, weather, orientation, inverter and loss inputs. PVWatts exposes those fields because capacity alone does not determine yearly production. [1]
Recalculate the nameplate before reading the headline
Multiply the exact module count by each module’s rated watts, then divide by 1,000 to obtain kWdc. For example, a hypothetical 20-module layout using 420-watt modules is 8,400 watts, or 8.4 kWdc. That arithmetic verifies only the DC nameplate represented by the equipment list. It does not say how many kilowatt-hours the array will produce, how much electricity the home uses, how the utility credits exports or what a bill will be. Keep the unrounded equipment calculation beside any rounded marketing label.
Read annual kWh as a model result
PVWatts describes itself as an estimator for the energy production of grid-connected photovoltaic systems. Its current interface identifies DC system size, module and array type, system losses, tilt, azimuth, DC-to-AC ratio, inverter efficiency, solar resource data and other inputs. [1] Ask for the actual report or export used by the proposal—not a screenshot containing only an annual total. A second proposal with the same kWdc can reasonably produce a different estimate if roof planes, shade, weather data, equipment assumptions or loss treatment differ.
Keep a small unit audit on the worksheet
Create separate rows for module count, watts per module, calculated kWdc, inverter AC rating, modeled first-year kWh and selected household annual kWh. Label every unit. A useful derived comparison is modeled annual kWh per kWdc, but treat it as a diagnostic ratio, not a performance promise. If two otherwise similar proposals show materially different ratios, use the difference to locate an input question. Do not “correct” one result by multiplying capacity by an invented number of sun hours.
Match the model to the roof drawing
A blended annual total can hide that different roof planes use different tilt, azimuth and shade assumptions. Count modules on each plane, identify north on the drawing and ask whether the model represents planes separately when their orientations differ. DOE notes that solar output and potential savings depend in part on roof direction, sunlight, system size and household consumption. [2] A larger array placed on a less productive plane is not automatically a better comparison than a smaller, well-documented layout.
Separate the production estimate from household coverage
Compare modeled solar kWh with the reconciled usage file only after both records are stable. A proposed annual-energy figure near the home’s annual consumption does not mean every month or hour will match, that the utility bill disappears or that the system provides outage power. The home can import at one time and export at another, and tariffs and fixed charges remain utility-specific. Use “annual modeled production divided by selected annual consumption” only as a labeled planning ratio.
Freeze the version that supports the contract
Save the final module schedule, roof layout, PVWatts or equivalent input report, annual result and proposal revision together. If module count, equipment, plane assignment or site findings change, require an updated kWdc calculation and production model. DOE’s homeowner guidance emphasizes comparing the home’s circumstances and project choices rather than relying on a universal solar answer. [2] This record does not guarantee a future year’s weather or operation; it makes the estimate reproducible enough for a homeowner to ask the right questions.
Before your next conversation
- Recalculate count × module watts into kWdc.
- Record inverter AC rating separately.
- Save every production-model input and the annual kWh result.
- Compare roof planes and production per kW as diagnostics.
- Re-run the model after a material design change.
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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