The comprehensive site for designing PV/Solar Electric Systems

Step 0 - Determine the Solar Hours of your location

The term for solar radiation striking a surface at a particular time and place is called insolation and the on an average total insolation striking the earth on a clear day is about 1000 watts per square meter. Many factors determine how much sunlight is available at a given site including atmospheric conditions, the earth's position in relation to the sun and obstructions at the site. Other factors that affect performance of a PV system include conditions like dust, which obstructs sunlight falling on the PV Panels and temperature conditions of the site which affect PV Panel performance based on the technology used in the panels. Refer to section of PV Panels for more information on affects of atmospheric temperature on PV Panel efficiency. It is important to take into account these conditions which contribute to loss of efficiency of the system when designing the overall rating of the system. PEAK SUN HOURS or SOLAR HOURS at a given site are the number of hours per day when the solar insolation equals 1000 watts per square meter. In the US, the National Renewable Energy Labs, NREL, has done extensive studies of the SOLAR HOURS around the USA and collected data over many years. They have published the 'average' solar hours at all major metro areas and locations around the country at their website: http://rredc.nrel.gov/solar/pubs/redbook/. Information on Solar Hours around the world can be found at: http://wrdc-mgo.nrel.gov

In the northern hemisphere where the sun is predominantly in the southern sky, solar panels should point towards the southern sky to collect the maximum solar energy. The sun's height above the horizon is called altitude which is measure in degrees above the horizon. When the sun is at its highest altitude for the day, the time is called solar noon. A location's latitude determines how high the sun appears above the horizon at solar noon throughout the year. As a result of earth's orbit around the sun with a tilted axis, the sun is at different altitudes above the horizon at solar noon throughout the year.

The highest average insolation will fall on a collector with a tilt angle equal to the latitude. As a general rule the following outlines the optimum tilt angle of a PV Array for different seasons: For a fixed tilt angle year round long chose the angle to equal the latitude of the location. If you can change the tilt angle, set the angle to be Latitude + 15degrees in the winter and the angle to be Latitude - 15 degrees in the summer.

Based on your geographical location, determine the average "Solar Hours" and the tilt angle of your PV Panels.

In the northern hemisphere there is a huge difference between the "Solar Hours" in the Summer months and the Winter months which determines the energy output of the PV system. When sizing the system it is important to design the system to be capable of supporting the loads during the months when the amount of solar insolation is the lowest. The advantage of a Grid Interactive system is that the loads can be supported by drawing power from the Grid during the winter months when the solar insolation is at its lowest. The trade offs to be considered when designing a grid interactive system is to choose between the number of hours/days the system needs to support critical loads without drawing power from the grid and how reliable the power from the grid is which will allow an overall smaller PV system to support loads throughout the year while drawing power from the grid as required.

Shading critically affects PV array's performance. Even a small amount of shade on a PV panel can reduce the panels performance by 70-80%. Minimizing shading is an important element of choosing where to place the PV Array. Shading can occur from trees, structures, poles and wires. PV Arrays should not be placed close to chimneys or other structures on the roof, for roof mounted arrays. A solar site analysis device, such as the Solar Path Finder iconis highly recommended to help determine the shading of a particular site.

There are essentially 2 PV Panel mounting methods employed in the industry, Fixed tilt arrays and tracking mount arrays. Tracking mount arrays are further classified in two categories: Passive Tracking and Active tracking arrays.

Fixed tilt installations can be Rack mounted, pole mounted or BIPV (building integrated PV panels) also referred to as direct mounting. They can be ground mounted or roof mounted.

Tracking mount installations are typically ground mounted and used in large installations to optimize the performance on the system. Home owners looking to install PV systems on their roofs typically do not use tracking systems as they are more expensive.

It is important to note that PV Panel performance degrades as panel temperatures rise and so when they are mounted on the roof make sure there is enough space between the rack and the roof to allow air flow around the panels to help reduce operating temperatures.

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