A solar panel is nothing more than a collection of solar cells on
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
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
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.
- 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
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
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
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.