Solar System Design Calculator

Solar system sizing uses energy use before roof area

Monthly kWh tells the calculator how much electricity the system is trying to offset. Roof area, panel wattage, tilt, shade, inverter selection, losses, and degradation then determine whether that target can fit the site. Use peak sun hours as an estimating input, not a production promise. For a site-specific production model, compare the calculator result with NREL PVWatts using the project's address, array type, tilt, azimuth, losses, DC/AC ratio, and inverter assumptions.

For the rest of the solar path, use the inverter sizing calculator to check string limits, the Solar Panel Output Chart to document production assumptions, and the solar planning hub when you want the full workflow in one place.

Recommended inputs

Formula path

Target monthly solar kWh = monthly usage x target offset

Target daily solar kWh = target monthly solar kWh / 30

Required kWdc = target daily solar kWh / peak sun hours / performance ratio

Panel count = ceiling(required kWdc x 1000 / panel wattage)

First-year production = rounded system kWdc x peak sun hours x 365 x performance ratio

Year-25 production = first-year production x (1 - annual degradation) ^ 24

Capacity factor = first-year production / (rounded system kWdc x 8,760)

How to use the sizing result

The output shows both the theoretical size and the rounded panel count. Real systems are built from whole modules and must still be checked against inverter input limits, roof layout, electrical code requirements, structural requirements, and utility interconnection requirements.

A location with stronger solar resource can produce more kWh from the same array size than a shaded or lower-resource site. The calculator keeps the performance ratio visible by deriving it from system losses, so users can adjust losses for soiling, temperature, inverter conversion, wiring, mismatch, shade, and practical availability.

Worked example

A home uses 900 kWh/month and the user wants to offset 80% of that load. With 4.8 peak sun hours per day, 20% system losses, a 0.80 performance ratio, 0.5% annual degradation, and 420 W modules:

Target monthly solar = 900 x 80% = 720 kWh

Target daily solar = 720 / 30 = 24 kWh/day

Required kWdc = 24 / 4.8 / 0.80 = 6.25 kWdc

Panel count = ceiling(6.25 x 1000 / 420) = 15 panels

Rounded system size = 15 x 420 / 1000 = 6.3 kWdc

First-year production = 6.3 x 4.8 x 365 x 0.80 = about 8,830 kWh

Year-25 production at 0.5% degradation = about 7,829 kWh/year

What this page does not claim

• It does not replace a site survey or equipment datasheets.
• It does not approve roof structure, utility interconnection, or code compliance.
• It does not model seasonal output or location-specific production without a site-specific resource model.
• It does not replace PVWatts, NSRDB weather data, or a site-specific production proposal.
• It does not replace inverter input checks or final design review.