Photovoltaic cells are the main ingredient of all photovoltaic solar energy systems, turning sunlight directly into electricity. Since the first silicon photovoltaic cells were discovered in the 1950s, technological developments have improved performance and transformed solar energy capabilities. An increasingly viable and popular renewable energy source, grid-connected photovoltaic systems grabbed a 75 percent share of U.S. installations in 2009, up from 31 percent in 2001.

Being grid-connected means that a solar photovoltaic system is linked directly to the available electrical grid or utility lines. A connected system comprises a photovoltaic panel or set of panels that directly convert sunlight into electricity. The size of the system dictates how independent from the grid it’s possible to be. Grid-connected systems need inverters to convert direct current into alternating current usable electricity. The inverter connects to the main electrical supply with a meter showing the number of generated kilowatt hours. Excess electricity can be sold back to the grid.

Grid-connected photovoltaic systems guarantee a home always has access to power, even if the solar energy fails or is insufficient. The system’s inverter connects to a battery bank that can store energy to be used in a power failure. An advantage of grid-connected systems is that they are not dependent on the sun shining. Off-grid photovoltaic energy is only intermittent but grid-connected systems ensure that any additional electricity needed is automatically delivered by the grid.

More than 35 states now have “net metering” legislation, meaning that householders benefit from selling excess energy back to the grid at normal retail prices. The Department of Energy notes that this puts people in Hawaii and New York at a great advantage because of the high retail electric rates. State and federal incentives include sales tax exemptions on the systems themselves, property tax exemptions, and personal state income tax credits.

As well to lower energy bills, grid-connected solar systems can add overall value to properties too. A federal study of California homes revealed that a 3,100-kilowatt hour system increases the home’s value by an average of $17,000.

Installing solar systems near existing utility power lines means that photovoltaic generated electricity costs more than that supplied by conventional methods. At its cheapest, solar energy is estimated to be 25 cents per kilowatt hour — generally between double and quadruple the cost of utility-supplied electricity. Initial costs are high so in practice installing a grid-connected solar system is like paying your electricity bill up front, years in advance.

The permits and legal requirements for installing grid-connected systems can be costly, complex, and time-consuming. While national standards are beginning to be adopted, many regional agencies have different legislative procedures so the best starting point is the local city or county building department.

Of the three basic solar panel types–monocrystalline, polycrystalline and amorphous–monocrystalline is the most efficient in collecting solar energy and therefore somewhat more effective in regions with low sunlight. As the name suggests, they are made from a single large silicon crystal cut from an ingot. Polycrystalline panels use many small crystals to form the collection surface, while amorphous, or thin film, solar panels consist of silicon particles applied to the surface of large plates. Monocrystalline panels, while more efficient, are only slightly so. They are also the most expensive of the three types.

Some manufacturers, including industry leader Sanyo technologies, have combined monocrystalline and amorphous thin film to produce a hybrid panel that Sanyo has dubbed Heterojunction with an Intrinsic Thin Layer (HIT). According to Sanyo’s marketing literature, these hybrid panels “boast high conversion efficiency ranging from 15.3 to 16.4, excellent temperature characteristics, and considerable output under diffuse and low light conditions.” Sanyo’s 190-watt photovoltaic (PV) module has earned a 17.4 percent efficiency rating, well above the industry average of 12 percent.

Industry experts consider solar panels with a collection capacity of greater than 100 watts to be high-powered. The wattage of a panel describes the amount of power the panel can produce in full sunlight at 77 F. The selection of high-powered panels compatible with the typical household 12-volt system dwindles as the wattage soars upward because the highest powered panels are designed for grid-tie systems rather than stand-alone systems, which deposit the power in a battery storage bank. You must also keep in mind that two panels with lower wattage will add up to the same collection capacity and be less expensive. In the case of solar panels, bigger does not automatically equate to better.

The U.S. Department of Energy’s Efficiency & Renewable Energy Program (EERE), established to develop innovations in the solar panel field, is working to optimize solar collector efficiency. One example is an experiment using a MicroDish composed of a concentration of Spectrolab solar cells–ultra-high-efficiency cells–in which EERE tested the use of mirrors designed to multiply the sun’s power. This application is intended “to substantially increase the viability of PV for cost-competitive applications.”