Sunlight can be converted into electricity by using photovoltaic (PV) cells, also known as solar cells. A PV module is composed of many interconnected PV cells. Each PV cell is made out of layers of semiconducting materials. When light falls on the cell some of the light is absorbed by the semiconductor. The light absorbed is energy that knocks electrons in the material loose and allows them to flow freely. This flow of electrons is current that can be drawn from the PV cell and used.

http://micro.magnet.fsu.edu/primer/java/solarcell

Photvoltaic (PV) systems are measured in kilowatts (kW). A kW is a unit of power. A kilowatt-hour (kWh) is a unit of energy. A car's odometer and speedometer is a good example for demonstrating the relationship between energy and power. A car's speedometer is similar to a PV system's kW power output display in that it shows the rate at which work is being accomplished at a specific time, the speed of the car as compared to the kilowatts produced by the PV system. This is a measure of power. The odometer is similar to a PV system's kWh meter in that it tracks the amount of work accomplished over a time interval, the distance covered as compared to the kilowatt-hours produced. This is a measure of energy. The relationship between both distance and speed, and kW and kWh, is linked by the time element:

Power (speed/kW) x Time (seconds/hours) = Energy (mph/kWh)

Another way to understand the relationship between kW and kWh is that if you burn a 60W lightbulb for 1 hour you will have consume 60Wh of energy. kWh is the commonly used billing unit for electrical companies.

The size of a PV system required for a home varies. There are many factors involved in calculating the size of a PV system but the two basic factors used are the electrical usage per month and the insolation (INcoming SOLar radiaTION) of the specific location. By looking at your utility bill you can determine your average monthly electrical usage which would be the amount of energy your PV system is looking to cover. The insolation of an area is the number of sun hours the area receives a day. Thus to calculate the approximate PV system size for your home:

PV System Size (appx.) = Average Electrical usage per day ÷ Insolation

The chart below is an example of the average insolation in three different U.S. cities and the approximate size of a PV system needed if the homeowner used around 3600kWh of electricity per year, an average of 10kWh per day. A house usually requres a 2-3kW PV system.

PV System Size in New York City = 10kWh ÷ 4.5h = 2.2kW

City	Average Insolation (sun-hours per day)	PV System Size (kW)
New York City	4.5	2.2
San Diego	5.5	1.8
Seattle	3.8	2.6

Please note that all numbers are approximate. To get actual PV system size the pitch of the roof must also be taken into account as well as any shading of the roof.

Annual Pattern of Insolation year in 1987
http://www.physicalgeography.net/fundamentals/7g.html