What is “PV” (photovoltaics), or solar electricity?
How can we get electricity from the sun?
What are the components of a PV system?
How does a residential solar electric system work?
How much electricity does a PV system generate?
Do PV systems work better in hot or cold weather?
Will my system work on cloudy days?
What happens during a power outage?
Does a solar PV system make hot water?
How can I tell a water heating panel from a solar electric module?
Can I generate heat for my home with a residential solar electric system?
What do the terms on-grid, grid-connected, grid-tied and off-grid mean?
How do I know if a residential solar electric system would work on my home?
How many solar panels do I need to produce enough electricity to run my house?
Will a system produce enough energy to cover all my electricity needs?
What other options do I have if the system cannot be mounted on my roof?
Do all states require utilities to offer net metering?
Are government incentives available for solar?
How much power will a residential system produce?
Can the modules withstand high winds and hail?
How long will my solar power system last?
How can I tell how my system is performing?
What maintenance does the system require?
Do I need permission from the local utility to connect my solar system to the grid?
Do I need approval from my homeowners’ association?
Can I have a solar electric system on my house and still be connected to city power?
Can I be totally independent from the utility?
What's the difference between PV and other solar energy technologies?
Here are some quick answers to many of the questions home & business owners frequently ask. If you still have a question after reading this page, please feel free to ask us.Use the contents list to quickly navigate through the page, or just read all of it one by one.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
Many, but not all, states require utilities to offer net metering, but the size and technology requirements vary.
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.
The amount of power produced by a system varies depending on the size of the system, your geographic location and climate.
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).
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.
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.
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.
Yes. Your installer will know how to obtain the necessary permits from your local government.
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.
If you belong to a homeowners’ association, consult your covenants for details. Many states prohibit homeowners’ associations from restricting solar devices.
Yes. In fact, most systems are designed as on-grid systems, meaning they are designed to interconnect with utility power.
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.
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.
On any given day the solar radiation varies continuously from sunup to sundown and depends on cloud cover, sun position and content and turbidity of the atmosphere. The maximum irradiance is available at solar noon which is defined as the midpoint, in time, between sunrise and sunset. Irradiance is the amount of solar power striking a given area and is a measure of the intensity of the sunshine. PV engineers use units of watts (or kilowatts) per square meter (w/m2) for irradiance. Insolation (now commonly referred as irradation) differs from irradiance because of the inclusion of time. Insolation is the amount of solar energy received on a given area over time measured in kilowatt-hours per square meter (kwh/m2) - this value is equivalent to "peak sun hours." For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the sun shone for six hours with an irradiance of 1,000 w/m2. Peak sun hours corresponds directly to average daily insolation given in kwh/m2. Many tables of solar data are often presented as an average daily value of peak sun hours (kwh/m2) for each month. Insolation varies seasonally because of the changing relation of the earth to the sun. This change, both daily and annually, is the reason some systems use tracking arrays to keep the array pointed at the sun. For any location on earth the sun's elevation will change about 47° from winter solstice to summer solstice. Another way to picture the sun's movement is to understand the sun moves from 23.5° north of the equator on the summer solstice to 23.5° south of the equator on the winter solstice. On the equinoxes, March 21 and September 21, the sun circumnavigates the equator. For any location the sun angle, at solar noon, will change 47° from winter to summer. The power output of a PV array is maximized by keeping the array pointed at the sun. Single-axis tracking of the array will increase the energy production in some locations by up to 50 percent for some months and by as much as 35 percent over the course of a year. The most benefit comes in the early morning and late afternoon when the tracking array will be pointing more nearly at the sun than a fixed array. Generally, tracking is more beneficial at sites between 30° latitude North and 30° latitude South. For higher latitudes the benefit is less because the sun drops low on the horizon during winter.