Harnessing sunlight to power our lives
Solar electric systems power a vast array of items, from credit card-sized calculators to communications satellites, highway traffic signs, and homes, schools, and warehouses. Also called photovoltaic (PV) systems, they work by converting light energy into electricity. “Photo” comes from the Greek word for light, while “voltaic” refers to the production of electricity.
The basic building blocks of this kind of solar system are PV cells. They are small, thin, square-shaped pieces of a semiconductor material, most often silicon. When light hits a cell, it generates a small amount of electricity, typically 1 or 2 watts. Linked together in modules, cells can produce large amounts of electricity.
Components of a Typical PV System
Most residential PV systems consist of these components:
- PV modules usually consist of lightweight panels with metal frames and a glass surface that protects the PV cells from rain, snow, and hail.Modules are typically about two to four feet wide and four to six feet long, and produce from 60 to 200 watts of electricity. In recent years, however, manufacturers have developed other configurations, like solar shingles that do double duty as roof shingles (more about that later).Modules are wired together to form a solar array. Arrays range in size from about 120 square feet for the smallest systems to 1,000 square feet or more for larger systems. The array’s size depends on a number of factors, including how much electricity you want to produce. For more about sizing a PV system, see the article “Is a Solar Electric System Right for You?“
- Mounting structures prevent the wind from carrying away the PV modules that are attached to them. They also hold the modules at an angle to receive the maximum amount of sunlight. Most mounting structures are simple steel racks bolted to the roof in a fixed position. Modules may also be mounted on poles or other types of structures on the ground. Tracking devices can be added that tilt the panels in response to the position of the sun. These trackers can increase output by as much as 25%. By adding moving parts with electronic controls, though, they also may increase the cost and decrease the reliability of the system. If you need more power, it may be more cost effective to add a few more panels instead of trackers.
- Inverters take the direct current (DC) electricity produces when light hits a solar cell and convert it to the alternating current (AC) needed by homes. Most residential systems wire all the modules to one boxlike inverter mounted on the side of the house or in the garage. At least one company has begun manufacturing micro-inverters that attach directly to individual modules; the company says their micro-inverters will increase the PV system’s overall efficiency. The additional benefit of micro-converters is that if something happens to one converter, the rest of your system will continue to function.
- Some systems include a bank of large batteries that store electricity. Most people who buy PV systems skip the batteries because they add thousands of dollars to the system’s cost. Read on to learn more about the pros and cons of battery backups.
On or Off the Grid?
In remote regions, an “off-the-grid” or independent PV system may be less costly than running wires to connect with the utility grid. With these off-the-grid systems, the home is powered entirely by PVs, with a bank of batteries storing excess electricity for use when the sun isn’t shining.
Most residential PV installations are connected to the local utility’s electricity grid. This gives them a source of backup electricity when the sun isn’t shining and, depending on state laws, allows them to feed any excess electricity into the grid. A home’s electricity meter tracks the outputs and inputs. In the more than 35 states with “net metering” laws, you pay only for the electricity you used above what your system produced over a 12-month period.
Unfortunately, in some states net metering laws also stipulate that the utility company need not pay you for surplus electricity you may have generated over the same period. For this reason, many people size their PV system so that it won’t produce more than their expected annual electricity use.
Another consideration for those on the grid is whether you want the security of having back-up power during grid outages. If so, you’ll need to add batteries to your system. When outages occur, utility companies require automatic shutdown of your power and everyone else’s-for the safety of utility line workers.
In addition to the first-generation systems described above, other PV systems are now available that integrated into the building instead of something sitting on top of it; some even take the place of a building material like roof tiles or windows. These products, known as building-integrated photovoltaics or BIPV, haven’t been around as long as conventional PV modules, so if you go this route, it’s wise to do more homework to make sure you’re getting a quality product that meets your needs.
- Solar shingles. These look a lot like ordinary roof shingles and become a part of the roof. They cost more than regular PV panels but if you’re planning to reroof, solar shingles start to look more affordable.
- PV laminates. Designed to fit between the raised seams of metal roofs, these thin PV sheets have a “peel and stick” adhesive backing. You can also buy metal roofing material with the PV laminates pre-applied at the factory. PV laminates cost about the same as traditional PV panels, but installation is a lot less labor intensive if you have a metal roof.
- Solar glass. Used as skylights, windows, awnings, canopies, and even walls, solar glass (also known as solar or PV glazing) is made with semi-transparent PV cells laminated between panes of glass. It generates electricity while still allowing a limited amount of visibility and light transmission. Right now solar glass is very expensive compared to traditional solar panels but look for prices to come down as more products become available.