A responsible system design starts with annual electricity use and models how a specific system may perform on your property.
Begin with annual energy consumption
Look for the total kilowatt-hours used during the most recent 12 months. Annual usage is more reliable than multiplying one bill because heating, cooling, travel, and seasonal weather can change consumption significantly.
Estimate the production target
Your desired offset is the portion of annual electricity use the system is designed to produce. The right target depends on utility export rules, available roof area, budget, and future changes such as an electric vehicle, pool, heat pump, or home addition.
Account for real-world production
Two homes with the same usage may need different system sizes. Local sunlight, panel orientation, roof pitch, shade, temperature, equipment efficiency, and normal system losses all influence expected production.
- Panel wattage and module efficiency
- Roof orientation, tilt, and shade
- Inverter type and system losses
- Local weather and solar resource
- Utility interconnection limits
Convert system size into panel count
Once a designer estimates the required system capacity in kilowatts, panel count is generally calculated by dividing system watts by the wattage of the selected panel. For example, equipment choice can change panel count even when the total system capacity remains similar.
- Panel count = desired system watts ÷ panel wattage
- Fewer high-wattage panels can produce a similar nameplate capacity to more lower-wattage panels
- Final layout must fit required roof setbacks and usable roof planes
Ask for a production report
Before comparing proposals, request the estimated first-year production, the annual usage used in the design, the proposed offset, and the assumptions for shade and degradation. Compare kilowatt-hours and total system size—not panel count alone.