solar Panels

A solar panel is nothing more than a collection of solar cells on a single panel.  Each individual solar cell in the solar panel can generate approximately half a volt of current so if you combined 36 cells you would get around 18 volts of current. The panels usually have light aluminum frames to hold the solar cells in place and are covered with a non-reflective glass to protect the cells from weather and damage since silicon cells are very fragile. Solar panels vary in size and in electric output.  In general, the more solar cells on each panel the more watts of electricity they can produce. 

The output of a solar panel is usually stated in watts.  The amount of watts of electricity generated by the panel is determined by multiplying the rated voltage by the rated amperage. The formula for wattage is:

          VOLTS x AMPS = WATTS

Let's use as an example a large solar panel measuring about 37.5 inches by 61.8 inches that might be used in a typical home energy system.   The solar panel has a rated voltage of 26 volts and a rated  amperage of 6.9 amps.  The wattage calculation would look like this:

         26 volts x 6.9 amps = 180 watts

If a particular location has an average of 6 hours of peak sun per day, then the solar panel in this example can produce an average 1080 watt-hours (6 x 180)  of power per day or a little over 1 kilowatt-hour per day.  Most homes use far more than one kilowatt-hour per day.  Most use between 10 and 25 kilowatt hours per day. Given this it is going to take a lot more than one solar panel to generate enough electricity to completely power your home.  For a homeowner running on 20 kilowatt hours per day it would take approximately 19 panels to provide 100% of the electricity they need on a daily basis.  That is a lot of solar panels. Many homeowners do not have enough space on their south facing roof for this many panels.  Consequently, in most home PV applications where you are connected to the grid you should think of the system as providing part, but not all of your energy.  Whenever you receive a bid from a solar contractor they should tell you what percentage of your energy consumption the system is likely to produce.

Types of Solar Panels

There are a number of different types of solar panels manufactured today. Briefly, they are:

• Mono-crystalline - These types of solar panels uses solar cells which are made from a very pure single large crystal, cut from ingots. They are the most efficient type of solar panels but are also the most expensive. Their performance is somewhat better in low light conditions (but not as good as some advertising hype would have you believe).  Overall efficiency on average is about 12-15%. Most panels of this type are warranted for 20-25 years. They are usually blue-grey in color and have a fairly uniform consistency.
Some of the major manufacturers of mono-crystalline solar panels are Sharp, Kyocera and BP.


• Bifacial Mono-crystalline - A new type of solar panel has recently emerged on the market which uses mono-crystalline solar cells but which has glass on both sides so that it can collect energy from both sides of the solar panel.  By collecting light from both sides the bifacial panels have higher efficiency for about the same cost.  Efficiency levels up to 20% have been reported for these types of panels.  Typically these panels are installed in a pole mounted solar array so that ambient light can reach the panel from both the front and the back.  They can also be effective if roof mounted on a roof that has a white matt or has been painted white to allow light to reflect on to the back of the panel.  Because these panels are fairly new there is not a lot of information yet on their durability but most are warranted for 20-25 years, the same as for traditional panels.  Bi-facial panels are a particularly attractive solution for pole mounted systems since a given pole mount can usually only hold 9-12 solar panels.  By using more efficient panels the cost tradeoff of the panels versus the cost of the tracking system is improved. Currently Sanyo is the leading manufacturer of bifacial solar panels.
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• Poly-crystalline Block - With most poly-crystalline solar panels the silicon in the solar cells is cast from large blocks of silicon which may contain many small crystals. Some manufacturers use a slightly different approach for creating poly-crystalline solar cells.  Currently, poly-crystalline solar panels are the most common.  They are slightly less efficient than single crystal, but once set into a frame with 35 or so other cells, the actual difference in watts per square foot is not much.  Poly-crystalline cells look somewhat like shattered glass and have a dark blue to almost black color. Overall efficiency on average is about 11-13%.


Poly-crystalline String Ribbon - String ribbon photovoltaics use a variation on the polycrystalline production process, using the same molten silicon but slowly drawing a thin strip of crystalline silicon out of the molten form between two strings. These strips of photovoltaic material are then assembled in a panel with the same metal conductor strips attaching each strip to the electrical current. This technology saves on costs over standard polycrystalline panels as it eliminates the sawing process for producing wafers. Some string ribbon technologies also have higher efficiency levels than other polycrystalline technologies. Overall efficiency on average are from 11-14%. Evergreen is the primary provider of string ribbon solar panels.


• Amorphous - Amorphous solar panels are also referred to as "thin film" solar panels.  In these types of panels the silicon is spread directly on large plates, usually of something like stainless steel. The thin film type of solar cells can also be spread on to more flexible plastic materials to make very flexible solar panels.  These types of solar cells are much cheaper but also much less efficient than mono crystalline or poly-crystalline solar panels.  Therefore in order to provide as many watts as the other types of solar panels they must be much bigger in size.  However, because they can be put on to flexible backings they have proven very valuable in certain types of applications where flexibility is more critical than power.  For example, these types of solar panels are often used in portable products such as solar backpacks and solar bags. Overall efficiency on average is about 5-6%.



• Concentrating Photovoltaic Solar Panels -  These types of panels employ a lens or mirror to concentrate the sun's energy on to the individual solar cells. In theory these types of panels will be more efficient because by concentrating the sun's energy fewer solar cells are needed to create the same amount of energy. Many of the concentrating panels use a type of plastic lens, called a Fresnell lens, to concentrate the sun's energy. Another type of concentrating solar panel called a Heliotube uses a series of troughs which track the movement of the sun to provide greater solar exposure to the solar cells.  Concentrating solar panels reduce the amount of photovoltaics needed to produce electricity, and also reduce the amount of space needed for a photovoltaic installation. Their main disadvantage is that they depend solely on direct light to produce electricity, while stand-alone photovoltaic panels can use both direct and diffuse light. Many regions do not receive enough direct light throughout the year for these systems to make these types of panels practical. Another disadvantage is their complexity of their construction, which makes these systems more difficult to build and install than conventional PV panels. Concentrating panels are also considerably heavier than conventional PV panels and have a number of moving parts which makes them more susceptible to failure than conventional panels. These types of panels are not widely used in residential solar PV systems.



• Group III-V Technology Panels - Currently there is a great deal of research targeted at creating very advanced solar cells and panels.  Solar cells created with these types of advanced technologies are often referred to as Group II and IV cells. These sophisticated solar cells use a variety of materials with very high conversion efficiencies to capture more of the light spectrum. A typical material used in this technology is gallium arsenide, which can be combined with other materials to create semiconductors that can respond to different types of solar energy. Though these technologies are very effective, their current use is limited due to their very high costs. They are currently employed only in space applications for use with satellites or lunar rovers such as the Mars rovers. Group III-IV solar panels can have efficiencies as high as 25%.

In most home energy applications the homeowner will probably want to go with either mono-crystalline or poly-crystalline solar panels.  Which of the two really depends upon price.  Because roof space on a south facing roof is at a premium for most home PV applications we don't usually don't recommend thin-film amorphous panels.  Amorphous solar panels have to be much larger to provide as much as electric output other types of panels and on most roofs there is not enough space to generate as much power as you need.

Combining Solar Panels into a Solar Array

In most home solar installations the solar panels will be combined into a grouping known as an array. There are several ways of connecting the panels depending upon what you want to accomplish from an electrical point of view. Solar panels can be wired in series or in parallel to increase voltage or amperage respectively. Series wiring refers to connecting the positive terminal of one panel to the negative terminal of another. The resulting outer positive and negative terminals will produce voltage which is the sum of the two panels, but the amperage stays the same as one panel. So two 12 volt/3.5 amp panels wired in series produces 24 volts at 3.5 amps. Four of these wired in series would produce 48 volts at 3.5 amps. 

When solar panels are wired for use with a grid-tied systems they are often wired in series in order to produce a fairly high voltage before going to the inverter.  The greater the voltage the smaller the wire which is needed to transmit the current, so by wiring the panels in series to increase the voltage it is not necessary to use a large gauge wire. This makes the panels easier to wire and reduces costs since heavy gage wire is very expensive.

Parallel wiring refers to connecting positive terminals to positive terminals and negative to negative. The result is that voltage stays the same, but amperage becomes the sum of the number of panels. So two 12 volt/3.5 amp panels wired in parallel would produce 12 volts at 7 amps. Four panels would produce 12 volts at 14 amps.

When using off-grid systems or grid-tied with battery backup it is common to have all or part of the panels wired in parallel in order to keep the voltage low.  This makes it easier to interface to a battery system which is usually 12, 24 or 48 volts.  Sometimes a mix of series and parallel wiring is used.  In the example below the four panels use a mix of parallel and series wiring.