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PV technology converts sunlight directly into electricity. It works any time the sun is shining, but more electricity will be produced when the light is more intense (a sunny day) and is striking the PV modules directly (when the rays of sunlight are perpendicular to the PV modules). Unlike solar systems for heating water, PV technology does not use the sun's heat to make electricity. Instead, PV produces electricity directly from the electrons freed by the interaction of sunlight with semiconductor materials in the PV cells. If you are interested in learning more details about how solar cells work www.howstuffworks.com/solar-cell.htm is a good site to visit. But you don't need to understand the detailed physics of how PV works to understand its appeal: investing in PV allows you to produce your own electricity with no noise, no air pollution, and no moving parts while using a clean, renewable resource. A PV system will never run out of fuel, and it won't increase our oil imports from overseas. In fact, it may not even contribute to the trade deficit, because many PV system components are manufactured in the United States. Due to these unique characteristics, PV technology has been called "the ultimate energy source for the 21st century."
A PV system that is independent of the grid typically consists of a battery bank and charge controller. This type of system, a) in the following graphic, can be used to provide direct-current (DC) power, or with an inverter, can be used to supply power for alternating current (AC) loads. A PV system tied to the utility grid, b) in the following graphic, typically consists of one or more PV modules connected to an inverter (or power conditioner) that changes the system's DC output to AC, which is compatible with the utility grid. You may include batteries in the system to provide reliable back-up power in case your utility experiences power outage.
The graphic below illustrates typical arrangements for (a) a direct current off-grid, and (b) an alternating current grid-connected, residential PV system.
PV systems produce power intermittently because they work only when the sun is shining. This is not a problem for PV systems connected to the utility grid, because additional electricity you need is automatically delivered to you by your utility. Systems that are independent of the grid use battery banks to provide power when the sun is not out. PV-generated electricity is usually more expensive than conventional utility-supplied electricity. Improved manufacturing has reduced the cost to less than one percent of what it was in the 1970s, but the cost (amortized over the life of the system) is still about 25 cents per kilowatt-hour. This is about two and a half times the retail price that Vermont residents now pay for electricity from their utilities. Financial incentives can help make PV more affordable, but it can't match today's price for electricity from your utility. Unlike electricity purchased month by month from a utility, PV power comes with a high initial investment and no monthly charge thereafter. This means that buying a PV system is like paying years of electric bills up front. You'll probably appreciate the reduction in your monthly electric bills, but the initial expense may be significant. By financing your PV system, you can spread the cost over many years, and incentives can also lighten your load. Click here for more information on financing for renewable energy systems.
Photovoltaic technologies are modular and systems can be designed to meet a wide range of electric loads from powering watches and calculators to providing utility-scale electric power supply. Sometimes, for those new to the subject, the terminology can be a bit confusing. For descriptions of the most common types of systems click on headings in the Learning More table below.
For component descriptions click on headings in the Learning More table below.
Off-Grid
Home A typical off-grid home will have a solar array, a battery bank, an inverter, and a number of other components needed for system control, monitoring, and safety. Off-grid home systems often include additional energy sources for use during extended periods of cloudy weather. These may include renewable sources such as a windmill or hydroelectric generator or a fossil fuel generator .
Off-grid system costs will vary widely. Small systems designed to meet very limited energy needs (for example a small cabin lighting or small appliance system) can be installed for as little as $1,500. Larger systems, with the capacity to run conventional loads in an energy efficient home, can cost $15,000 or more. Grid-Tied
Home with Battery Storage
The battery system may be sized to provide enough power only for priority emergency uses, or for more typical consumption patterns. Of course, increasing the storage and backup capacity of the system tends to increase the price. When batteries are fully charged, and the sun is shining, the homeowner can take advantage of net metering and sell power back to the utility company at retail rates. Typically, you can expect costs for medium household sized systems (2 kiloWatt to 4 kiloWatt) with battery storage backup to be in the $20,000 to $40,000 range. Grid-Tied
Home without Battery Storage As there is no method of storing energy with this type of system, when utility power fails there is no back up power and the house is without electricity. It is not possible to directly use the power from the PV array when grid-power fails. Stand
alone: (e.g. solar fence charger, water pumping)
Modules
Modules or arrays, by themselves, do not constitute a PV system. We must also have structures on which to put them and point them toward the sun, and components that take the direct-current (dc) electricity produced by the modules or arrays and condition the electricity so it can be used in the specific application. These structures and components are referred to as the balance of system (BOS).
Batteries Several factors can be used to help determine the size of the battery bank. These include the electric load, the duration of required reserve power, and the availability of a source of backup power (grid or generator). A good quality, lead-acid battery bank will last from 500 to 1,000 charge-discharge cycles depending on depth of discharge and attention to maintenance considerations. Other types of batteries are available such as Nickel Cadmium. These batteries are longer lasting, but quite a bit more expensive than lead acid batteries. A battery box is needed to enclose the battery bank. The box contains potential acid spills, keeps out unfamiliar persons, and keeps objects from falling on the batteries possibly damaging or shorting battery terminals. The battery box must also provide adequate ventilation of explosive hydrogen gas (produced during battery charging) to the outside.
This includes careful attention to charge and discharge levels, periodic watering (except in the case of gel-cell batteries), and inspection of cables and connections for tightness and corrosion. Charge
Controller Mounting
Hardware
Metering Many PV owners are interested in more specific information on the output of their PV system. Many systems offer options for direct system metering, using either a computer hook-up, or other meter installation. Off-grid homes, or systems with battery back-ups often also have a meter system to monitor battery voltage, charge and discharge levels, battery reserve capacity, power used, and historical battery data. These meters can be set up for automatic monitoring by the system installer. A good battery monitor is a very useful diagnostic and customer service tool. Generator Wiring,
Fuses, Disconnect Box |
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Solar
Electric Basics 
What is a solar electric, or photovoltaic,
system?
