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Solar PV - Sizing

Choosing the size of a solar photovoltaic system depends on the type of system desired. Grid-tied systems are easier to size, as a customer can choose to offset part or all of their electrical usage. Off-grid systems need to provide all of the electrical demands, plus a ‘buffer’ for periods of inclement weather. Many off-grid systems utilize a hybrid system with a backup generator, but relying on this type of electrical generation is expensive.

To size an off-grid system, you will first need to inventory your electrical demands or loads. The simple act of turning on the light switch or starting the microwave draws electrical demands that need to be accounted for. Typically, the load estimation will account for all electrical loads present, the amount of time each day the appliance is used, and the number of days each week the appliance is used. An example of a Load Estimation Worksheet may be found at www.ext.colostate.edu/energy/solar.html.

To size a grid-tied system, electrical customers can add up their total electrical usage for the year (available on their utility bills), and divide by 365 days per year. This daily electrical usage (kWH) is then divided by the average solar resource, or insolation in their area. In Colorado, an average insolation factor is 5.5 hours per day year-round. The result is the amount of kilowatts needed in a solar array not accounting for design inefficiencies. To conservatively estimate the solar array size and account for inefficiencies, you can then divide the kilowatts by 0.77 to determine the array size. A qualified solar system designer can best size your system based on your unique situation.

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Updated Thursday, May 17, 2012