A photovoltaic system, also photovoltaic power system, solar PV system, PV system or casually solar array, is a power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and directly convert sunlight into electricity, a solar inverter to change the electrical current from DC to AC, as well as mounting, cabling and other electrical accessories to set-up a working system. It may also use a solar tracking system to improve the system's overall performance or include an integrated battery solution, as prices for storage devices are expected to decline. Strictly speaking, a solar array only encompasses the ensemble of solar panels, the visible part of the PV system, and does not include all the other hardware, often summarized as balance of system (BOS). Moreover, PV systems convert light directly into electricity and shouldn't be confused with other solar technologies, such as concentrated solar power (CSP) and solar thermal, used for both, heating and cooling. PV systems range from small, roof-top mounted or building-integrated systems with capacities from a few to several tens of kilowatts, to large utility-scale power stations of hundreds of megawatts. Nowadays, most PV systems are connected to the electrical grid, while stand-alone or off-grid systems only account for a small portion of the market. Operating silently and without any moving parts or environmental emissions, PV systems have developed into a mature technology that has been used for fifty years in specialised applications, and grid-connected systems have been operating for over twenty years. A roof-top system recoups the invested energy for its manufacturing and installation within 0.7 to 2 years and produces about 95 percent of net clean renewable energy over a 30-year service lifetime. As the growth of photovoltaics fits an exponential curve, prices for PV systems have rapidly declined in recent years. However, they vary by market and the size of the system. In 2013, prices for rooftop systems in the United States and Japan were just below $5.00 per watt, while in the highly penetrated German market, prices were close to $2.00. Nowadays, solar panels account for less than half of the system's overall cost, leaving the rest to installation labor and to the PV system's remaining components.