Sizing A PV Solar Array

1. Determine what your KWh electric consumption is (check your utility bill or call your local utility company). Then match PV production to your electric consumption.

For example:

If you consume 600 kilowatt-hours per month (KWh/month) and want to produce 100% of your electricity with a PV system with no battery backup, do this equation to calculate; 600 KWh/month x 12 months equals 7,200 KWh/year or approximately 20 KWh/day.

Most of the U.S. has 3.5 to 5 Sun Hours of solar input. This means that a 1 kilowatt AC PV system in a 4.5 Sun Hour region will produce 4.5 kilowatt-hours per day. 20 kWh/day divided by 4.5 sun hours equals 4.4 KW AC.

Go to "PVWATTS" at: http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/ and enter your location, 4.4 KW, your roof tilt and orientation. See your monthly and annual estimated PV production for a 4.4 KW system or any other size system.

2. Match your PV array size to your roof space. You need full sun on your solar array all day (from at least 9am to 4pm). Trees, chimneys, vents and other buildings can block the sun or make array installation difficult. Square footage examples; 225 sq. ft. array (Qty of 32 - 70 Watt modules) / 140 sq. ft. array (Qty of 20 - 75 Watt modules).

3. Match your PV system cost to your budget. PV modules are about half of the total system cost. The other equipment you may need depending on your system configuration are mounting hardware, combiner boxes, disconnect switches, power center, charge controller, inverter, battery bank and wiring.