GRID-TIED SYSTEMS
Modern grid-tied inverters let you keep the reliability of the utility, but allow you to produce your own, pollution-free energy with renewables.
While many of us fantasize about freeing ourselves from a lifetime of utility bills, the reality is that the grid is usually reliable and accessible. Most of us are already on-grid, so taking our homes off-grid doesn't make much environmental or economic sense.
Batteryless grid-tied systems are also known as on-grid, utility-interactive, grid-intertied, or grid-direct. They generate solar electricity and route it to the loads and to the electric utility grid, offsetting a home's or business's electricity usage.
Living with a grid connected solar electric system is no different than living with utility electricity, except that some or all of the electricity you use comes from the sun. The drawback of these batteryless systems is that they provide no outage protection when the utility grid fails.
The inverters on these types of systems actively "test" the grid and make operating decisions based on grid conditions. These inverters need the grid present to operate (except when they are installed in an AC coupled system). While "testing" if the grid voltage or frequency goes outside UL1741 specifications, the inverter immediately disconnects itself from the house electrical system and from the grid. This process is extremely quick and therefore, the inverter will recognize the problem and disconnect itself. Once the grid is within specification again for five continuous minutes, the inverter reconnects and resumes operation.
Grid-tied systems are based on the principal of net metering, which allows homeowners to sell clean power to the utility or obtain a credit on their account when their system produces a surplus and to draw on utility power when it's needed. Grid-tied systems typically have no battery backup, making them less complicated to install and operate. Even though homeowners remain tethered to their local utilities, they don't have to worry about running short of power or about calculating precisely how much electricity they will consume.
With an off-the-grid system, every amount of energy counts, not only how much power the system can produce, but also in how the electricity is allocated. If the electrical load is greater than the PV modules can produce, power is drawn from a bank of batteries, an expensive and integral part of an off-the-grid system.
With grid-tied systems the utility grid becomes a means of "storing" your excess of electricity and acts as a backup, supplying electricity any time your demand exceeds the output of your system.
Anytime the sun is shining your solar system provides electricity to your home. If the system produces more electricity than you're using, the surplus flows onto the utility lines that supply your home, making your meter run backwards. The monetary value of the energy is counted and accumulates as credit, which can be used when the loads are higher and the PV system output is lower or at zero, such as at night or during cloudy weather. The utility meter on your home keeps track of any electricity you feed back onto the grid, as well as what you use. This is calculated depending on net metering regulations where you live.
It is possible to produce enough electricity from your grid-tied system that you get a credit from your utility company instead of a bill. This is what is called a Net Zero home. Most utilities allow credits to be carried over from month to month, which allows homeowners to draw on summer's "banked" electricity during the winter months, when system production tends to be lower.
According to the Solar Energy Industries Association (SEIA), in a typical residential net-metered solar installation, only 20% to 40% of the energy produced by the PV system is exported to the grid--most of the energy produced is used directly in the home. Customers are only billed for their net energy use--that is, the electricity they use at home beyond their PV system's production and beyond any accrued credits. Surplus PV electricity is usually consumed close by, reducing the load on the utility grid, reducing transmission and distribution losses, and reducing emissions and pollution from power plants.
