What
is "PV" (photovoltaics), or solar electricity? Back
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The
word itself helps to explain how photovoltaic (PV) or solar electric
technologies work. First used in about 1890, the word has two parts:
photo, derived from the Greek phos, which means light, and volt, a
measurement unit named for Alessandro Volta (1745-1827), a pioneer
in the study of electricity. Photovoltaics literally translated is
light-electricity - just what photovoltaic materials and devices do;
they convert light energy to electricity, as Edmond Becquerel and
others discovered in the 18th Century.
How
can we get electricity from the sun? Back
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When
some semiconducting materials, such as certain kinds of silicon, are
exposed to sunlight, they release small amounts of electricity. This
process, known as the photoelectric effect, is the emission or ejection
of electrons from the surface of a metal in response to light. It
is the basic physical process in which a solar electric or photovoltaic
(PV) cell converts sunlight to electricity.
Sunlight is made up of photons, or particles of solar energy. When
photons strike a PV cell, they may be reflected or absorbed, or they
may pass right through. Only the absorbed photons generate electricity.
When this happens, the energy of the photon is transferred to an electron
in an atom of the PV cell (which is actually a semiconductor).
With its newfound energy, the electron escapes from its normal position
in an atom of the semiconductor material and becomes part of the current
in an electrical circuit. By leaving its position, the electron causes
a hole to form. Special electrical properties of the PV cell—a
built-in electric field—provide the voltage needed to drive
the current through an external load.
What
are the components of a PV system? Back
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The
components of a PV system include PV modules (groups of PV cells),
which are commonly called PV panels; an inverter for a utility-grid-connected
system and when alternating current (ac) rather than direct current
(dc) is required; wiring; and mounting hardware or a framework. For
an off-grid or stand-alone system a charge regulator or controller
and one or more batteries are required.
How
long do PV systems last? Back
to Top
A
PV system that is designed, installed, and maintained well will operate
for more than 25 years. The basic PV module (interconnected, enclosed
panel of PV cells) has no moving parts and can last more than 30 years.
The best way to ensure and extend the life and effectiveness of your
PV system is by having it installed and maintained properly.
How
does a residential solar electric system work? Back
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Solar
cells in the modules mounted on your roof convert sunlight directly
into DC power. A component called an inverter converts this DC power
into AC power that can be used in your home. The system is interconnected
with your utility. During the day, if your solar system produces more
electricity than your home is using, your utility may allow net metering
or the crediting of your utility account for the excess power generated
being returned to the grid. Your utility would provide power as usual
at night and during the day if your electricity demand exceeds that
produced by your solar system. Systems are also available with a battery
backup. Part of the power produced by your solar system during the
day is used to charge the batteries, which provide power for your
critical loads in the event of a power outage.
For tracking structures that follow the sun across the sky or fixed
arrays, the annual energy production is maximum when the array is
tilted at the latitude angle; i.e., at 40°N latitude, the array
should be tilted 40° up from horizontal. If a wintertime load
is the most critical, the array tilt angle should be set at the latitude
angle plus 15° degrees. To maximize summertime production, fix
the array tilt angle at latitude minus 15° degrees.
How
much electricity does a PV system generate? Back
to Top
In
most areas of the United States, a 10% efficient PV system will generate
about 180 kilowatt-hours per square meter. A PV system rated at 1
kilowatt will produce about 1800 kilowatt-hours a year. Most PV panels
are warranted to last 25 years or more and to degrade (lose efficiency)
at a rate of less than 1% per year. Under these conditions, a PV system
could generate close to 36,000 kilowatt-hours of electricity over
20 years and close to 54,000 kilowatt-hours over 30 years. This means
that a PV system generates more than $10,000 worth of electricity
over 30 years.
Do
PV systems work better in hot or cold weather? Back
to Top
A
PV module's power output is reduced at high temperatures, but the
lifetime of the PV module (estimated to be over 30 years) is not affected
by normal heat. The duration and the intensity of the sunlight has
a major effect on the output of a PV module, and the increase in temperature
has a lesser effect on the output. A general "rule of thumb"
for crystalline silicon PV modules (the most common type to date)
is that the efficiency is reduced about 0.5 percent for every degree
C increase in temperature. PV modules are usually rated at module
temperatures of 25°C (77°F) and seem to run about 20°C
over the air temperature. So on your hot day of 100°F, the module
will be 120°F or 50°C, so it will have its power reduced by
12.5 percent.
The design of a PV system usually takes into consideration the need
to allow some "convective cooling" for the PV modules, that
is, some way to passively dissipate the heat generated from the module
and minimize the module temperature to increase the performance.
Will
my system work at night? Back
to Top
No,
sunlight must be present for your solar modules to produce power.
You will continue to draw power though. In a grid-intertied system,
you draw power from your utility at night. In a battery-backup, off-grid
application, your batteries will charge during the day and you will
draw power from them at night.
Will
my system work on cloudy days? Back
to Top
Yes,
though they produce less electricity. Just as it is advised that we
wear sunscreen on a cloudy day, solar panels will gain a significant
amount of power on a cloudy day. Under a light overcast sky, panels
might produce about half as much as under full sun.
What
happens during a power outage? Back
to Top
There
are two types of solar residential systems. One type of system powers
your home during daylight hours, but does not provide power in an
outage, even on a sunny day. Another type of system powers your home
during daylight hours, but also has a battery backup designed to provide
power to your home’s critical loads during an outage, day or
night.
Does
a solar PV system make hot water? Back
to Top
No.
People often confuse PV panels with solar thermal panels that involve
water circulating through tubes to be heated by the sun for swimming
pool water heating. PV modules convert sunlight into electric current
to operate appliances, motors, pumps and other devices.
How
can I tell a water heating panel from a solar electric module? Back
to Top
Solar
electric modules are typically one to two inches (2.5 to 5 cm) thick
with 32 or more three to four inch (7.5 to 10 cm) blue or black solar
cells on the back of the cover glass. Solar water heating panels are
generally much thicker and may have tubes connected to a flat black
plate under the glass, or a black tank inside the collector panel.
Can I generate heat for my home with a residential solar electric
system? Back
to Top
Not
directly. PV solar power systems are designed to provide electricity
to run your lights, appliances and other electric devices in your
home. Other solar technologies are designed to turn the sun’s
light into heat instead of electricity.
What
do the terms on-grid, grid-connected, grid-tied and off-grid mean?
Back
to Top
On-grid,
grid-connected or grid-tied means connected to the utility electrical
grid. Off-grid refers to systems that are not connected to the utility
electrical grid.
How
do I know if a residential solar electric system would work on my
home? Back
to Top
Solar
electric power works for most homes. Systems can be engineered to
work with most roofing materials, in most locations where direct sunlight
is available, in almost every region of the United States. You need
a sunny place on your roof about 120 square feet and up to 1,000 square
feet for larger systems. Shading from trees can reduce the efficiency
of a specific installation, and may be required to prune in order
to qualify for funding. A south-facing roof area is optimal, but solar
electric panels can be mounted on west- or east- facing roofs and
still produce better than 90 percent of the power of a true south
roof mounting.
How
big are the panels? Back
to Top
Each
solar module is approximately 5 feet (1.5 meters) long and 21⁄2
feet (0.75 meters) wide. The modules are usually grouped in a set
of four to make up an array.
Why
is shade a problem? Back
to Top
Because
of the wiring design of a solar module, all of the individual solar
cells on a module must receive full sunlight for the module to work
properly. If any portion of the module is shaded, the entire module
power output-even those sections still exposed to sunlight-is lowered.
What
if I need a new roof? Back
to Top
This
is the ideal situation for installing solar. Before laying the roof,
you can install flashable mounting brackets that provide the highest
level of protection from leakage. If you already have a system and
need to replace the roof several years later, the panels can be removed
and replaced back on the new roof without much cost.
How many solar panels do I need to produce enough electricity to run
my house? Back
to Top
That
depends on your average electrical usage, climate, roof angle, shading
problems and other factors. To approximate the array size you need,
multiply your average daily electrical demand in kilowatt-hours by
0.25. The result is the approximate size of solar array, in kilowatts,
needed to meet your electrical demand.
Will a system produce enough energy to cover all my electricity needs?
Back
to Top
It
is possible to install a system that eliminates your electric bill,
but a solar electric system does not need to provide all of the electricity
you need to be of great value. A small system that displaces an average
of one-quarter to one-half of your average demand reduces your electric
bill and provides a great value as both a financial investment and
contribution to society by reducing pollution.
What
other options do I have if the system cannot be mounted on my roof?
Back
to Top
If
a roof-mounted system proves impractical for you, a ground-mount,
trellis or pergola application may be a more suitable option.
Under
net metering, any excess electricity produced by your solar energy
system is delivered back into the utility grid, effectively spinning
your meter backwards. Your meter spins forward when your solar energy
system is not producing all of the electricity you are currently using.
Your electric meter keeps track of this net difference as you generate
electricity and take electricity from the utility grid.
Do
all states require utilities to offer net metering? Back
to Top
Many,
but not all, states require utilities to offer net metering, but the
size and technology requirements vary.
Are
government incentives available for solar? Back
to Top
Yes.
State agencies and municipal utilities offer rebate and incentive
programs for homeowners and small businesses to promote the installation
of renewable energy equipment such as ours. Incentives can cut the
cost of your system in half or more, saving you thousands of dollars.
Your installer can tell you more about the incentives available in
your area.
How
much power will a residential system produce? Back
to Top
The
amount of power produced by a system varies depending on the size
of the system, your geographic location and climate.
Can
the modules withstand high winds and hail?
Back
to Top
Absolutely.
The panels are supported by our roofer-designed mounting system that
has been tested to withstand 125 mph winds and can work on almost
every type of roofing material. Most modules can withstand one inch
(2.5 cm) hailstones at 50 mph (80.5 kph).
How
long will my solar power system last? Back
to Top
With
no moving parts and made of very inert materials, solar modules are
tough and many have warranties of 25 years. They are manufactured
to withstand a golf ball sized hailstone hitting at 50 mph. They are
expected to last much longer than the warranty. Most of the related
system components also last for many years without problems.
How
can I tell how my system is performing? Back
to Top
We
provide a power meter with the system. The power meter shows you exactly
how much electricity you generate and use, and when you send power
back to the utility grid.
What
maintenance does the system require?
Back
to Top
Your
system should last for years without any problems. For battery back-up
systems, batteries may need replacement every five to 10 years. If
it is convenient, you can hose off the modules two or three times
a year.
Do
I need a building permit? Back
to Top
Yes.
Your installer will know how to obtain the necessary permits from
your local government.
Do
I need permission from the local utility to connect my solar system
to the grid? Back
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Yes.
The local utility has rules and procedures that are required to connect
any generator to the grid safely and legally. These rules are generally
based on national standards. Your installer will help you with the
procedures.
Do
I need approval from my homeowners’ association?
Back
to Top
If
you belong to a homeowners’ association, consult your covenants
for details. Many states prohibit homeowners’ associations from
restricting solar devices.
Can
I have a solar electric system on my house and still be connected
to city power? Back
to Top
Yes.
In fact, most systems are designed as on-grid systems, meaning they
are designed to interconnect with utility power.
Can
I be totally independent from the utility?
Back
to Top
Yes,
if you have an optional battery back-up system installed. There is
an additional charge for that type of system which requires extra
components and batteries.
What's
the difference between PV and other solar energy technologies?
Back
to Top
There
are four main types of solar energy technologies:
•
Photovoltaic (PV) systems, which convert sunlight directly to electricity
by means of PV cells made of semiconductor materials.
• Concentrating solar power (CSP) systems, which concentrate
the sun's energy using reflective devices such as troughs or mirror
panels to produce heat that is then used to generate electricity.
• Solar water heating systems, which contain a solar collector
that faces the sun and either heats water directly or heats a "working
fluid" which is used to heat water.
• Transpired solar collectors, or "solar walls," which
use solar energy to preheat ventilation air for a building.
