Photovoltaic systems use semiconductor technology to convert sunlight directly into electricity. The systems are simple and quiet, and require no moving parts. Batteries store energy for use when the sun is not shining. Photovoltaic systems come in a near infinite number of sizes, ranging from a single solar cell to power a calculator or a single module (containing multiple cells) to power a light; to multiple modules to power a water pump or a home; to large arrays of modules to provide industrial-scale power. Photovoltaic technology is well established and field proven, and many sizes and types of modules are commercially available from a number of different companies. 

Photovoltaic systems are especially well suited to locations where accessing an electrical grid is either not feasible or expensive. In many such locations, photovoltaic technology is the least-cost option for meeting remote energy needs. Photovoltaics has proven to be a reliable source of power in an ever-growing number of applications. Lighting is one common use for these systems. Cost-effective applications of lighting powered by photovoltaics include small garden lights, street lights, lighting for recreational areas, highway signs, warning signs and signals, and for businesses and homes both in the developed and developing world. Photovoltaics is ideal and commonly used for water pumping because water can be pumped into a storage tank during daylight hours, then distributed by gravity whenever it is needed. Photovoltaic systems commonly pump water for remote livestock watering tanks, and in the developing world, entire village water supplies are powered by photovoltaics. Other uses include remote monitoring, refrigeration, and energy for small commercial ventures. Virtually any power need can be met with photovoltaics, although some are more cost-effective than others.

You should use a PV system if it operates better and costs less than alternatives.  The cost of energy produced by PV systems continues to drop. However, kilowatt-hour for kilowatt-hour, the cost of PV energy is still generally higher than energy bought from your local utility. Also, the initial cost of PV equipment is higher than an engine generator. Yet, there are many applications where a PV system is the most cost-effective long-term option. The number of installed PV systems increases each year because their many advantages make them the best option. Consider the following issues:

• Site Access - A well-designed PV system will operate unattended and requires minimum periodic maintenance. The savings in labor costs and travel expense can be significant.
• Modularity - A PV system can be designed for easy expansion. If the power demand might increase in future years, the ease and cost of increasing the power supply should be considered. 
• Fuel Supply - Supplying conventional fuel to the site and storing it can be much more expensive than the fuel itself. Solar energy is delivered free.
• Environment - PV systems create no pollution and generate no waste products.
• Maintenance - Any energy system requires maintenance but experience shows PV systems require less maintenance than other alternatives.
• Durability - Most PV modules available today are based on proven technology that has shown little degradation in over 15 years of operation. 
• Cost - For many applications, the advantages of PV systems offset their relatively high initial cost. For a growing number of users, PV is the clear choice.

System designers know that every decision made during the design of a PV system affects the cost. If the system is oversized because the design was based on unrealistic requirements, the initial cost is increased unnecessarily. If less durable parts are specified, maintenance and replacement costs are increased. The overall system life-cycle cost (LCC) estimates can easily double if inappropriate choices are made during system design. Don't let unrealistic specifications or poor assumptions create unreasonable cost estimates and keep you from using this attractive power source. As you size your PV system, be realistic and flexible.

That depends on your application. Generally, the cost of PV energy is higher than energy bought from your local utility. However, if you need power in a location not served by a utility, PV may be the cost-effective option. The number of PV system installations is increasing rapidly. As more people learn about this versatile and often cost-effective power option, this trend will accelerate.
You should consider your goals and distinguish between your wants and your needs. Reevaluate your ideas about having electric power available during all kinds of weather - 100 percent availability. Availability has a unique meaning for a PV system because it depends not only on reliable equipment but on the level and consistency of sunshine, and the capability of the energy storage system. Because the weather is unpredictable, designing a PV system to be available for all times and conditions is expensive, and often unnecessary. PV systems with long-term availabilities greater than 95 percent are routinely achieved at half the cost or less of systems designed to be available 99.99 percent of the time. Designing for lower availabilities decreases the size of the PV array and batteries and will save many dollars. 
Another way to resolve the availability issues is to design a Hybrid system which will include another energy source. 
Although saving money is important you should be determined to design and install a safe system that will last 25 years or more. Quality may cost more initially but will save money in the long run. 

All life on earth is supported by the sun, which produces an amazing amount of energy. Only a very small percentage of this energy strikes the earth but that is still enough to provide all our needs. A nearly constant 1.36 kilowatts per square meter (the solar constant) of solar radiant power impinges on the earth's outer atmosphere. Approximately 70% of this extraterrestrial radiation makes it through our atmosphere on a clear day. In the southwestern United States, the solar irradiance at ground level regularly exceeds 1,000 w/m2. In some mountain areas, readings over 1,200 w/m2 are often recorded. Average values are lower for most other areas, but maximum instantaneous values as high as 1,500 w/m2 can be received on days when puffy-clouds are present to focus the sunshine. These high levels seldom last more than a few minutes. The atmosphere is a powerful absorber and reduces the solar power reaching the earth at certain wavelengths. The part of the spectrum used by silicon PV modules is from 0.3 to 0.6 mirometers, approximately the same wavelengths to which the human eye is sensitive. These wavelengths encompass the highest energy region of the solar spectrum.
Talking about solar data requires some knowledge of terms because 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". Peak sun hours is defined as the equivalent number of hours per day, with solar irradiance equaling 1,000 w/m2, that gives the same energy received from sunrise to sundown. In other words, 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. Therefore, 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 months.
For tracking (structures that follow the sun across the sky by various mechanisms, thereby increasing the energy captured from the sun) 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.
Using inaccurate solar data will cause design errors, so you should try to find accurate, long-term solar data for your system location. These data are becoming more available, even for tilted and tracking surfaces. Check local sources such as solar system installers, universities, airports, or government agencies to see if they are collecting such data or know where you might obtain these values. If measured values on a tilted surface are not available, you may use the modeled data here. Data for fixed and single-axis tracking surfaces at three tilt angles (latitude and latitude ±15°) are provided. Two-axis tracking data are given also, as well as a set of world maps that show seasonal values of total insolation at the three tilt angles. All data are in units of kilowatt-hours per square meter. This is equivalent to peak sun hours--the number of hours per day when the sun's intensity is one kilowatt per square meter. See the database on Solar Resource. 

A well-designed and maintained PV system will operate for more than 20 years. The PV module, with no moving parts, has an expected lifetime exceeding 30 years. 
Experience shows most system problems occur because of poor or sloppy installation. Failed connections, insufficient wire size, components not rated for dc application, and so on, are the main culprits. The next most common cause of problems is the failure of electronic parts included in the Balance of Systems (BOS) - the controller, inverter, and protection components. 
Batteries will fail quickly if they are used outside their operating specification. In most applications, batteries are fully recharged shortly after use. In many PV systems the batteries are discharged AND recharged slowly, maybe over a period of days or weeks. Some batteries will fail quickly under these conditions. Be sure the batteries specified for your system are appropriate for the application. 

Maybe. However unless you are very handy or experienced in home wiring, we suggest using experienced professionals in the design and installation of anything more than the simplest of applications for the following reasons:

• You might void manufacturer's warranties
• You may not have a functional system after spending your hard earned money
• Electricity can be dangerous
• You may damage your home, or existing appliances

The goal of a stand-alone system designer is to assure customer satisfaction by providing a well-designed, durable system with a 20+ year life expectancy. This depends on sound design, specification and procurement of quality components, good engineering and installation practices, and a consistent preventive maintenance program.
System sizing is perhaps the easiest part of achieving a durable PV power system. A good estimate of system size can be obtained with the worksheets provided and the latest component performance specifications. The resulting system sizes are consistent with computer-aided sizing methods. Photovoltaic systems sized using these worksheets are operating successfully in many countries. 
Regardless of the method used to size a system, a thorough knowledge of the availability, performance, and cost of components is the key to good system design. Price/performance tradeoffs should be made and reevaluated throughout the design process. When you start your design, obtain as much information as you can about the components you might use. 
After studying all the issues, you should do an initial sizing of the PV system and get some ideas about specifying system components.