Calculate Your Daily Power Needs

• The amount of electricity you will need daily is a major factor in determining the number of solar panels you need to install. Your monthly power bill includes the number of kilowatt-hours you consumed during the whole month. Choosing a power bill that falls on a winter month and dividing the kilowatt-hours (kWh) by 30 should give your average daily consumption (kWh) during the months when you use the most amount of electricity. As an example, a bill showing 300 kWh consumed in March will give you an average of 10 kWh daily consumption (300 kWh divided by 30 days).
  
  

Know the Amount of Available Sunlight

• Knowing the amount of available direct sunlight in the area where you will mount the solar panels is another important factor that is used to determine the number of solar panels needed. Solar insolation maps such as those provided by the NREL, or national laboratory of the U.S. Department of Energy, provide information regarding the average number of hours sunlight is available in a given area. You can check with your local energy department, or visit the website of the U.S. Department of Energy to see copies of solar insolation maps covering your area.
  
  

Provide an Allowance for Power Loss

• Electrical current passing through wires, batteries and inverters suffer a certain degree of power loss. Adding an extra 25 percent on the total wattage required daily would compensate for any power loss. In the previous example, multiplying the 10 kWh daily power requirement by 125 percent should give you 12.5 kWh of electricity. This figure represents the 10 kWh daily power requirement plus the 2.5 kWh allowance for any power loss.
  
  

Calculate the Required Daily Power in Watt Hours

• Watt hours refer to the number of watts expended over a period of one hour. The daily power requirement in terms of watt hours is needed to calculate the number of solar panels in a solar energy system. The daily power requirement measured in watt hours can be derived by multiplying the number of daily kilowatt-hours by 1,000. In the previous examples, 12.5 kWh of electricity needed daily is equivalent to 12,500 watt hours (12.5 multiplied by 1,000).
  
  

Know the Daily Production Capacity of Each Panel

• One more factor needed to calculate the number of solar panels needed by your solar energy system is the total wattage that each panel can produce given the amount of available direct sunlight per day. To determine this factor, multiply the rated wattage of each panel by the number of available direct sunlight each day. As an example, a solar panel rated at 500 watts multiplied by 5 hours of available direct sunlight each day will give you a result of 2,500 watt hours of electricity.
  
  

Calculate the Number of Panels Needed

• Calculating the number of solar panels required to adequately provide your electrical power needs is a matter of dividing the total power requirement by the power output of each panel in terms of watt hours. In our examples, the power requirement is 12,500 watt hours of electricity per day. The power output of each panel is 2,500 watt hours of electricity per day. Dividing the required power of 12,500 watt hours per day by 2,500 watt hours of power produced by each panel per day will give you a result of 5 solar panels needed by the solar power system.