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System Size Considerations

Choosing the size of a system needed depends on the type of system. 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 (termed autonomy). Many off-grid systems utilize a hybrid system with a backup generator, but relying on this type of electrical generation is expensive and noisy. Hybrid systems can also utilize other renewable energy sources such as wind turbines as a secondary source of electrical generation.

Size An Off-grid System

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 (lights, appliances, etc.), the amount of time each day the appliance is used, and the number of days each week the appliance is used (weekend cabins do not use electricity every day, but have higher demands when occupied). By adding up the electrical load needs, inserting an autonomy factor, accounting for design inefficiencies (batteries can be 80 percent efficient, inverters can be 90 percent efficient), and any voltage drop due to long wire runs, a system designer can determine the size of system necessary for off-grid applications.

Size A Grid-Tied System

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 (insolation) in their area. In general, Colorado averages about 5.5 peak sun hours per day, with parts of the San Luis Valley experiencing 6 or more peak sun hours per day. The result is the amount of kilowatts (kW) needed in a solar array (not accounting for design inefficiencies). For example: a Colorado home had a total of 8,000 kWH of energy usage for a year:

  • 8,000 kWh/365 days = 21.9 kWh/day
  • 9 kWh/day ÷ 5.5 = 3.98 kW
  • 98 kW ÷ .90 inverter efficiency = 4.4 kW system

CSU Extension’s Solar Calculator can size a system for you if you know how much electricity you use in a given year.

Last updated: September 19, 2017 at 11:27 am