This will help you decide what size PV system would be required to eliminate your electric bill.  You can use this information and the amount of available space to get a rough idea of the size system you should get. 

 

Step 1. FIND YOUR MONTHLY AVERAGE ELECTRICITY USEAGE FROM    YOUR ELECTRIC BILL

This will be in kilowatt-hours (KWh).  Due to air conditioning heating, and other seasonal usage, it is a good idea to look at several bills.  You can add the typical summer, fall, winter, and spring bills and divide by four to find the average monthly usage.

Step 2.  FIND YOUR DAILY AVERAGE ELECTRICITY USE

Divide the monthly average number of kWh use by 30 (days)

Step 3.  FIND YOUR LOCATION’S AVERAGE PEAK SUN HOURS PER DAY
           
Follow the link to see the average peak sun hours.  For example, the average for Texas is between 3.5 and 5.5 peak sun hours.

Step 4.  CALCULATE THE SYSTEM SIZE (AC WATTS) TO PROVIDE 100% OF YOUR ELECTRICTY

Divide your daily average electricity use by the average sun hours per day.  For example, if the daily average electricity use is 30 kWh, and the site is in Fort Worth, then the system size would be 30 kWh/4.5h = 6.6kW AC

Step 5.   CALCULATE THE NUMBER OF PV MODULES REQUIRED FOR THIS SYSTEM

Divide the system AC watts in Step 4 by the maximum efficiency watt rating of the modules to be used, then divide by the inverter efficiency, usually 0.94 and you get the total number of modules required.  (You can round this number up)