Let's begin with some definitions and basics. Solar energy refers to the energy carried by electromagnetic radiation transmitted from the Sun. It can be used to produce heat, electricity and a chemical reaction. Sun's radiation is emitted obviously in all directions, and only it's very small portion is intercepted by the Earth. Numerically, this intercepted amount is characterized by so-called solar constant, which is the average amount of Sun irradiance that reaches the earth's upper atmosphere on a surface perpendicular to the sunrays. The value of this constant is approximately 1366 watts per square meter or 433 Btu/hr per square foot. Electricity produced from sunlight is casually called solar power. Note that technically speaking, power is energy per unit time, although casually both terms are often used interchangeably.


The radiant energy can be converted directly to electricity in photovoltaic (PV) cells. These cells are normally made of specially treated semiconductor with p-n junctions. Here is how it works. When a photon of light strikes the top of the semiconductor's wafer, it can penetrate through the p-n junction and free an electron. Free electrons can cross the junction into the n-type region and then held there, being unable to return back into the p-region. This creates a voltage of about 0.50-0.65 volts under open circuit (no-load) condition. When connected to a load, this voltage will create a flow of electric current. A typical commercial single-junction PV cell can produce from 0.5 to 5.0 watt depending on it size.
For a higher output, multiple cells can be connected in series and parallel and mounted together on a support structure to form panels (a.k.a. modules).

The characteristics of such modules are basically similar to those of the cells, only scaled up in voltage and/or current based on the number of cells and their connection scheme. Common small panels have 32 to 40 series-connected cells. They can produce up to 20 volt at open circuit, and about 16-17 V at peak load. Such voltage levels are suitable for charging 12V batteries and powering small inverters. Larger commercially available modules designed to generate solar power for homes have nominal voltages up to 100 volts and can include up to 60 cells. The multiple modules in turn can be interconnected in an array to produce higher voltages, currents and power levels. Parallel connection of the modules increases the maximum load current, series connection increases the output voltage, and the net wattage is increased in both schemes. Today's large-scale installations are configured for up to 600-1000 VDC. You may wonder what is the benefit of such a high voltage setup? The matter is that power is volts times amps. Therefore for the same wattage a higher voltage lets decrease an electric current level and lower energy loss in wires, which in turns allows smaller cables.


Photovoltaic arrays for residential PV generators are usually attached to the roof of the house. Normally they are installed on racks, which have to be bolted into the roof joists. For the highest net amount of daily sunlight, they should face true south if you are in the northern hemisphere, and true north if you in the southern hemisphere. The optimal tilt should be slightly lower than your latitude.
A large part of the sun's irradiance is absorbed and scattered by the Earth's atmosphere. By the time sunlight reaches the surface, its peak energy density drops to about 1,000 watts per square meter at noon on a bright day. Today's commercially available modules for home use deliver efficiency anywhere from 6 to 21%. This means that the peak output of a PV module at noon with an optimal tilt can be anywhere from 60 to 210 W/sq.m, or from 5.6 to 19 watt/sq.foot. Additional reflective devices can be used to concentrate more energy. Concentrated solar power is more efficient, but it is used primarily in commercial installations. The buyers are often asking about the cost of solar panels per square foot. This is not a very useful characteristic: with the same surface area different models will yield different output depending on their efficiency. That's why when choosing solar panels for the home you should rather consider two main things: the cost per watt and efficiency. For a reference, an average retail price of the panels is about $2/watt, with the lowest numbers being under $1/W.

A PV array produces DC voltage, which is then converted to AC by an inverter. The system wiring should comply with National Electric Code NFPA70® (particularly, with Article 690) and local codes.

Note that PV cells do not store energy- at night or on a rainy day they do not produce any electricity. Therefore, to provide continuous power in off-the-grid homes or emergency backup in grid-tie installations you need to store an excess of the generated energy in a battery bank. Another reason batteries are used is to supply surge currents to electrical loads. When choosing a battery, look for a deep cycle type for more storage capacity. A deep cycle battery is typically designed to allow repeated 80% depth of discharge. You don't need batteries if you are on grid and you want to use the system just to save on your energy cost, but not for emergency backup.

Utility-interactive photovoltaic systems must use special grid-tied inverters that meet UL standard 1741. These devices are synchronizing their voltage, frequency and phase with incoming AC voltage. This allows the system not only to energize your appliances but to feed any excess of PV-made electricity back to utility and help you defray the energy cost. A grid interactive inverter also includes an automatic AC line disconnect switch that prevents voltage backfeeding into unenergized utility lines. By the way, according to NEC®, there should be another DC disconnect before the inverter. For stand-alone PV powered loads that are not connected to your house wiring and for off-grid applications you can use less expensive stand-alone inverters. However, if you are connected to the utility AC line, you should never hook up any non-approved voltage source directly into your home's wiring. For more information see wiring diagram of a solar powered home generator.

At present, the main disadvantages of a PV system are its high initial price and relatively low efficiency. However, for remote locations a solar generator may be a better solution than extending electric lines, which would typically run for $20,000 to as much as $75,000 per mile.


Solar energy for your home: the basic facts.
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