General Terms

Photovoitaics (PV) is the direct conversion of light into electricity. Certain materials, like silicon, naturally release electrons when they are exposed to light, and these electrons can then be harnessed to produce an electric current. Several thin wafers of silicon are wired together and enclosed in a rugged protective casing or panel. PV panels produce direct current (DC) electricity, which must be converted to alternating current (AC) electricity to run standard household appliances. An inverter connected to the PV panels is used to convert the DC electricity into AC electricity. The amount of electricity produced ¡s measured in watts (W). A kilowatt (kW) is equal to 1,000 watts. A Megawatt (MW) is equal to 1,000,000 Watts or 1,000 Kilowatts. The amount of electricity used over a given period of time is measured in kilowatt-hours (KWh).


What is a solar rating?

The solar rating is a measure of the average solar energy (also called "Solar Irradiance") available at a location in an average year. Radiant power is expressed in power per unit area: usually Watts/sq-meter, or kW/sq-meter.

The total daily Irradiation (Wh/sq-meter) is calculated by the integration of the irradiance values (W/sq-meter). Click here for the Solar Radiance map of the USA.

Shading: If your solar collectors or solar panels experience any shading during the day the output of your solar energy system can be dramatically reduced. This is especially true of photovoltaic (PV) solar panels since a partially shaded PV panel can result in a loss of power across the PV array. Some solar incentive programs, such as the California Solar Initiative (CSI) reduce the incentive available to you if your solar system is impacted by any shading.

Solar Electric (Photovoltaic) System Calculations

Estimating Solar Electric (PV) System Size: Are of Solar Panels

On average (as a general "rule of thumb") modern photovoltaics (PV) solar panels will produce 8 - 10 watts per square foot of solar panel area. For example, a roof area of 20 feet by 10 feet is 200 square-feet (20 ft x 10 ft). This would produce, roughly, 9 watts per sq-foot, or 200 sq-ft x 9 watts/sq-ft = 1,800 watts (1.8 kW) of electric power.

Converting Power (watts or kW) to Energy (kWh)

One kilowatt-hour (1 kWh) means an energy source supplies 1,000 watts (1 kW) of energy for one hour. Generally, a solar energy system will provide output for about 5 hours per day. So, if you have a 1.8 kW system size and it produces for 5 hours a day, 365 days a year: This solar energy system will produce 3,285 kWh in a year (1.8 kW x 5 hours x 365 days).

If the PV panels are shaded for part of the day, the output would be reduced in accordance to the shading percentage. For example, if the PV panels receive 4 hours of direct sun shine a day (versus the standard 5 hours), the panels are shaded 1 divided by 5 = 20% of the time (80% of assumed direct sun shine hours received). In this case, the output of a 200 square-foot PV panel system would be 3,285 kWh per year x 80% = 2,628 kWh per year.

Estimating Solar Electric (PV) System Size to Replace a Specified Amount of Utility (grid) electricity

PV System Capacity Required (kW of PV) can be roughly calculated as follows:

Annual electricity usage = Monthly Usage x 12 months. Electricity usage is express in kilowatt hours (kWH)

KW of PV = (Annual Usage) / (78% x kWh/kW-year from Solar Radiance chart below x 365 days)

Energy production from a solar electric (PV) system is a function of several factors, including the following ... the "78%  used above assumes the following losses across the PV system: