Solar System Sizing and Production Estimates

Sustainability and Green Building
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After researching your local utility’s requirements surrounding interconnection and net metering, you will have a better sense of the maximum system sizes allowed given any restrictions currently in place set by the authority having jurisdiction or the utility. Some utilities do not allow more solar put into the electric grid than what the residence uses on an annual basis. Thus, some utility companies have a rule of thumb for maximum solar sizing.

For instance, California electric utilities require solar systems not to exceed 2 watts per square foot of conditioned space without an exceptional load justification. As another example, in Massachusetts, there is a "cap" on net metering for each utility for certain types of projects. Systems that are 10 kilowatts (kW) and under, however, are exempt from the cap, so many residential solar photovoltaic (PV) systems in Massachusetts are 10 kW or less.

Once you understand the basics of total system size, determined by panel wattage and the number of panels, you can start your design and solar PV production estimates. There are many solar calculators available to builders and the public. The PVWatts Calculator is the one most commonly used by solar providers and is available for free through the National Renewable Energy Laboratory. PVWatts calculates the performance of potential PV installations based on location, equipment and other site factors. It will determine the estimated annual solar production in kilowatt hours (kWh) based on the system size (in kilowatts), module type, roof orientation and shading. The tool also has the capacity to calculate savings based on local utility rates. You can learn about each input and output, and how to use the tool by clicking the “Help” button at the top of the PVWatts tool page.

As an example, the required solar load size in California is based on building electricity use, where the new code requires solar systems to offset 100% of electricity consumption for mixed-fuel homes. The new California Title 24, Part 6 building energy code also dictates that all-electric homes generate as much solar power as an equivalent mixed-fuel home would.

To calculate the size of the solar system to generate a predetermined percentage of electricity needed, there are various energy software programs available, including but not limited to:

Check which software is acceptable for each state energy code compliance by entering your state on the U.S. Department of Energy Building Energy Code Program’s website.