THE SHORT ANSWER
Kilowatts describe rated capacity; kilowatt-hours describe energy over time. A proposal should show both and explain the model connecting them.
Read the units before comparing
A module count multiplied by module watts produces a DC nameplate rating, commonly shown in kWdc. Annual production is an energy estimate shown in kWh. Similar-looking numbers are not interchangeable, and a larger nameplate does not by itself establish a proportionally larger annual result.
Identify the model inputs
NREL PVWatts estimates energy using array size, module and inverter assumptions, losses, tilt, azimuth and weather data. [1] Ask the proposal to preserve those inputs or an equivalent model report. A single annual kWh output without inputs cannot be reproduced or compared fairly.
Check the equipment schedule
Confirm module model, wattage and count, then recompute total DC watts. If the proposal rounds system size, retain the unrounded equipment calculation. Ask how alternate modules would change count, layout and nameplate capacity rather than accepting “equivalent” as a complete explanation.
Separate estimate from operation
DOE explains that generation depends on available sunlight and system conditions. [2] The modeled annual figure is not a weather forecast, utility-bill guarantee or proof of future equipment performance. It is a planning estimate that should carry assumptions and a version date.
Compare proposals on matched terms
Put kWdc, modeled kWh, model inputs and roof layout side by side. If production per kW differs materially, investigate orientation, shade, losses and inverter configuration. Do not choose solely by the largest annual claim; choose the design whose assumptions can be understood and verified.
Before your next conversation
- Distinguish kW from kWh.
- Recalculate module count times wattage.
- Save the production-model inputs.
- Investigate large production-per-kW differences.
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.
- 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.
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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