How rooftop solar panels make electricity

Solar panels turn sunlight straight into electricity. The basic unit is a photovoltaic, or PV, cell, a thin layer of semiconductor material often less than the thickness of four human hairs. When sunlight strikes it, some of the light's energy knocks electrons loose from their atoms, and that flow of electrons is electric current. A single cell is small and makes only about 1 or 2 watts, so cells are wired together and sealed between layers of glass or plastic to form a module, or panel. Panels connect into arrays, and the arrays connect to the house and the electric grid.

The parts of a home system

• Panels. They make the electricity. The more efficient a cell, the darker it usually looks, often deep blue or black.
• Mounts. Racks hold the panels at a fixed angle and have to stand up to wind, rain, hail and corrosion for decades. In the Northern Hemisphere, panels facing due south, tilted at the local latitude, make the most electricity over a year. Ground-mounted systems sometimes use trackers that follow the sun.
• Built-in solar. Some products build solar cells into roofing, windows or walls.
• Inverters. Panels make direct current, or DC, and the house runs on alternating current, or AC. An inverter converts one to the other. A system has either one central inverter or small microinverters on each panel. An inverter will likely need replacing at least once over a system's 25-year life.
• Batteries. Batteries store solar electricity for use at night or on cloudy days.

What cuts the output

• Shade. Microinverters let each panel work on its own, which helps where some panels may be shaded.
• Heat. Solar cells work best when they're cool. Heat lowers their voltage, and extreme heat can shorten their life.
• Reflection. Light bounced off the surface is lost. Anti-reflection coatings and textured surfaces help.

Most of the sunlight that reaches a cell is never turned into electricity. A cell's efficiency is the share of the solar energy striking it that becomes usable power.

Installing a system involves the roof, the wiring and the connection to the grid, so it's a job for a solar professional. Any work on the panels or the roof is too.

An annotated example

What to do next

If your house has solar panels, keep a record of when they were installed and what kind of inverter they use, since the inverter will likely need replacing during the system's life. Watch for trees that have grown to shade the panels, and leave any roof or panel work to a solar professional.