It seems almost magical that you can take a flat, glass-covered plate (a solar panel), stick it out in the sun, and get electricity to come out of it. It seems even more magical when you consider that a solar electric system, when broken down into its most essential elements, is little more than sand and wires. The real magic behind solar panels is a semi-conductor material (a natural mineral product) that converts light energy to electricity. Once you've produced the electrical energy, it's merely a matter of transmitting it via wires—from the solar panels and into your house or onto the electrical utility grid. Pure, simple, scientific magic.
Solar Electric System Basics
A home solar electric system is, quite literally, a household power plant. It generates electricity that is every bit as useful as the power you get from the utility, and, in fact, it can contribute power to the utility system. In this way, you can actually produce power not only for your own home but also for your neighbors' homes.
The heart of a solar power system is the solar panels, which can be installed on your roof or on the ground. A group of solar panels is called an array. When the sun is shining onto the array, the panels create electricity and feed it into wires that ultimately connect to your home's electrical service panel, your circuit breaker box. From there, the electricity can be used directly by the lights and appliances in your house. If your array is making more electricity that your house needs at any given moment, the excess electricity passes through a special electric meter that allows the power to move onto the utility grid, where it used by other utility customers.
Grid-Tied Home Solar Systems
A home solar system that is connected to the utility grid is known in the industry as "grid-tied" and is the most common, most simple, and least expensive type of home solar system. If you live in a developed area with standard electric service and you just want to offset some or most of your conventional energy use with solar electricity, you most likely will install a grid-tied system. The key distinction of grid-tied systems is that the energy flows both ways. When your house needs more electricity than your solar array is producing, the house pulls in power from the utility. When your solar production exceeds household demand, you feed the grid with your solar power (and you get credit for it, in one form or another).
So, grid-tied systems rely on the utility for extra power when needed. They also rely on the utility grid for operation. When the grid goes down (such as during a storm or utility repairs), grid-tied solar systems also stop working. The panels are still functional, of course, but the electricity flow is cut off before it gets to your breaker box or the utility meter. This is a safety feature to prevent household "power plants" from energizing the grid when the utility expects it to be shut down.
Other Types of Home Solar Systems
There are two other types of solar systems, in addition to standard grid-tied. The more common alternative to grid-tied is an off-grid system. This has most of the same components of a grid-tied setup, and it connects to the home's breaker box, but it does not connect to the utility grid. Instead, it includes a large collection of batteries, called a battery bank, that stores excess solar power during the day. When the sun goes down and the array stops generating electricity, the house draws stored power from the battery bank. The next day, when the sun comes out again, the array recharges the batteries, and the daily cycle continues.
Off-grid systems are common in remote areas where utility hookup is not available or is very expensive. They're also popular with hard-core solar (or renewable energy) enthusiasts who prefer to provide all of their own household energy. Off-grid systems cost about twice as much as grid-tied and require much more monitoring and maintenance, but they offer the reward of complete energy independence.
The third type of system is a combination of grid-tied and off-grid and is called, not surprisingly, grid-tied with battery backup. The name pretty much summarizes what it does. It's tied to the grid, and it has a battery bank for backup power when the grid goes down. Typically, the battery bank is relatively small and powers only a few "critical loads" in the house, such as the refrigerator, heating equipment, and some lights and outlets. It can also use the batteries to take advantage of off-peak utility rates, drawing grid power when it is relatively inexpensive. However, adding battery backup to a grid-tied system requires sophisticated and expensive equipment, not to mention complex design and installation as well as battery maintenance. For this reason, most homeowners opt for basic grid-tied systems instead.
The Nuts and Bolts of Grid-Tied Solar Systems
Solar electricity is technically called photovoltaic, or PV, which is a blending of the words photo (for light) and volt (for electricity). Sunlight contains units of light energy called photons. A solar panel converts that energy to electricity with its solar cells made of crystalline silicon, a semi-conductor—a material that turns one form of energy into electricity. Simply put, the photons of light strike one side of the solar cells; this excites electrons in the semi-conductor, and electricity comes out the other side. Conductive materials (essentially, wires) capture this energy and feed it the solar (PV) system.
Solar panels produce energy in the form of direct current, or DC, electricity. But your household appliances and lights run on alternating current, AC, power. Therefore, the DC from the solar panels must be converted to AC with a device called a DC-to-AC inverter. Some systems use a single inverter (called a string inverter), which converts the power from all of the solar panels; others use microinverters, which connect to one or two panels each and covert at each panel (or pair of panels).
Once the power is converted to AC, it goes through a disconnect switch (which is used to isolate the solar array from the rest of the system, for safety, repairs, etc.) then through a PV production meter, which keeps track of the solar energy produced. Finally, the power is fed into the home's breaker box, usually with a couple of special circuit breakers. At the other side of the breaker box is a net meter, which lets power run both onto and off of the grid, unlike a standard one-way meter, which lets power flow only from the grid to the house. The net meter records both incoming and outgoing power, so you and the utility company can track your household's net electricity use.
All grid-tied systems include a means for "rapid shutdown," or the ability to disable the system at the array. This enables emergency personnel (firefighters, etc.) to shut down a PV system from outside the house before entering it. Systems with string inverters include separate shutdown equipment, while those with microinverters have automatic shutdown capability built into the microinverters.