A solar panel consists of a set of solar photovoltaic modules that are electrically connected and mounted on a supporting structure. A photovoltaic module is a set of solar cells that have been assembled into a connected package. A solar panel can be used as part of a larger photovoltaic system that generates and supplies electricity for commercial and residential premises. Modules are rated by their DC output power under standard test conditions (STC), and usually range from 100 to 320 watts. The efficiency of a module determines the area of a module given the same rated output, therefore an 8% efficient 230 watt module is twice the area of a 16% efficient 230 watt module. A single solar module can produce only a limited amount of power. Most installations consist of multiple modules. A photovoltaic system usually consists of a panel or an array of solar modules, an inverter, and sometimes a battery and/or solar tracker and interconnection wiring.