How do I decide how big my system should be?

The size of your optimal solar array will be influenced by many variables. Before analyzing those variables, you should understand how solar is sized and measured.  The electrical capacity of solar panels is measured in watts (W). The typical solar panel is rated at 250-300 W. To get the total power (in watts) of your solar array, add together the wattages of each panel. Let’s say you had 10 300 W panels installed. The total wattage of your system would equal 3,000 W. 1,000 W is equal to 1 kilowatt (kW), so another way to describe the size of that system would be 3 kW. The average size of a solar array is 5 kW.

Your installer will estimate how many panels can fit on your roof given its footprint and shade susceptibility to determine the ideal size of your system. If the size of your roof is limited (meaning fewer panels can be installed), installers can compensate by offering high-efficiency panels. These panels will have a higher power rating (typically 300-350 W), and therefore will produce more electricity per panel. Installers will also use geospatial data to determine the optimal system size for your property, as roof orientation and climate factors will affect how much electricity your system produces. The final factor that will influence the size of your solar array is your project budget. Installers work closely with clients to maximize the amount of solar they install for the customer’s budget.

While sizing your solar array, installers will consider how much the solar electrical output will offset your electricity needs. While the power capacity of solar panels is measured in watts (or kilowatts), the amount of electricity produced by the panels is measured in watt-hours (or kilowatt-hours).

Source: National Renewable Energy Laboratory

You may recognize the term kilowatt-hour (kWh) from your electric bill. Utilities charge their customers based on how many kWh of electricity they consume each month. If you look at your utility bill from any billing cycle, you’ll be able to see exactly how many kWh of electricity your home or building consumed that month.  Each kW of solar you install will produce a certain number of kWh, which will directly offset your utility electricity consumption. The kW-to-kWh relationship varies with latitude and climate. Your installer will be able to accurately predict how many kWh of electricity your solar panels will produce each year.

To estimate how much solar you can install on your roof (and how much electricity it will produce each year), we recommend using the PV Watts tool. To get a sense of how much your solar will offset your electricity needs, divide the annual kWh production estimate by your annual kWh consumption of utility electricity (the sum of 12 monthly bills). View tutorial.

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