ABOUT PHOTOVOLTAIC SYSTEMS A photovoltaic (PV) module is usually smaller than a piece of plywood. The most common type contains a collection of individual PV cells made from silicon, each producing about 0.5 volt. The cells are protected by a layer of low-iron tempered glass in an aluminum frame. When sunlight strikes a cell, it causes the silicon to shed electrons, creating DC current. Balance of system. An inverter changes the DC into alternating current (AC), which runs appliances, lights and other electrical equipment. Most installations in the U.S. are grid-tied, meaning the house can draw power from either the utility grid or its PV array. When the PV cells aren’t producing enough power, the grid makes up the difference. When the array makes more than the house consumes, the electricity flows into the grid. Off-grid systems, which require a battery bank to store power, are more expensive and more complicated. Measuring power output. PV modules are rated by the amount of electricity they’re capable of producing, but actual performance may be less. The rated capacity is based on ideal circumstances, with the sun providing 1000 watts per square meter. Local weather conditions and site latitude can diminish or (more rarely) enhance PV module performance. Solar potential in the U.S. varies widely, from 7 kWh per square meter per day in parts of the Southwest to about 4 kWh per square meter in parts of the upper Midwest, New England and the Pacific Northwest. Where sunlight potential is lower, PV modules produce less electricity. The National Renewable Energy Laboratory produces a solar map with details for photovoltaic potential around the U.S. Cold days are best. The best PV weather is a very cold day when the atmosphere is crystal clear and the ground is covered with clean snow. A blue…