How Solar Production Is Estimated

Before you sign for a system, an installer estimates how many kilowatt-hours it will produce each year. EnergySage explains that these estimates come from design tools that model your roof and from calculators like PVWatts. This guide breaks down the factors and how to sanity-check an estimate.

The Factors That Drive Output

Production estimates start with the physical roof. EnergySage states that design tools incorporate satellite imagery to model the specific characteristics of a roof that affect electricity production, like shade, roof orientation, roof tilt, and geography.

Equipment choices matter too. EnergySage notes that panel efficiency, wattage ratings, and inverter type all feed the estimate, and that microinverters and power optimizers outperform standard string inverters. Total system size in kilowatts sets the overall scale.

The Tools Installers Use

Several software packages turn those inputs into a number. EnergySage names Helioscope, Aurora, and Solargraf as popular design software installers use to model a system.

There is also a free public option. EnergySage highlights PVWatts, developed by a national laboratory, as a free calculator that uses the same core factors of location, orientation, tilt, and system size. Homeowners can run it themselves to compare against an installer's figure.

Checking an Estimate With Production Ratio

One quick way to compare estimates is the production ratio. EnergySage defines it as annual kilowatt-hours divided by system size in kilowatts, which normalizes output so systems of different sizes can be compared.

The expected ratio depends on region. EnergySage reports ratios ranging from 1.0 to 1.3 in the Northeast to 1.4 to 1.8 in the Southwest, so an estimate far outside your region's range is worth questioning before you sign.