the presence, or lack thereof, of the electric utility grid. These systems are classified as either stand-alone or utility-interactive.
Stand-alone systems are those that do not have access to the electric utility grid, often referred to as off-grid systems. Typically found in remote locations, they generally use batteries to store energy produced by the PV array. Stand-alone systems normally use a traditional generator to charge batteries during times of the year when solar resources are minimal or when the array is unable to keep up with energy demands of the household. Generally more complex, these systems require more design and installation time than grid-direct systems.
Utility-interactive systems are those that do exactly that: interact with the electric utility power system. The power electronics used in these systems, namely the inverters, monitor the grid and react automatically to the conditions the grid presents. Utility-interactive systems can be broken down into two subcategories — grid-direct or backup systems. In commercial PV applications, the systems are nearly universally utility-interactive, grid-direct systems.
Tax incentives and electric utility rebate programs have made utility-interactive systems the largest market for PV to date. These systems are able to interconnect with and work in parallel with the electric utility grid. In a situation where the PV array is producing more power than what is being consumed on site, the inverter is able to push that power back onto the electric utility line, essentially running the customer’s meter backward and allowing the customer to sell energy to the electric utility. Then, as the sun goes down and more loads are turned on, the meter begins to run forward again, causing the customer to buy back energy from the electric utility. When the sun is up but the loads within the building are consuming more than the solar system is producing, the electric utility makes up the difference — but the meter is moving slower than it would if the PV array wasn’t present.
In utility-interactive, grid-direct systems, as shown in Fig. 1 (click here to see Fig. 1), there is no energy storage, and the PV modules are connected to an inverter, which inverts the DC power produced by the PV modules into AC power that matches the grid power. These systems have the advantage of higher efficiencies and are more simplistic in their design/installation compared to their backup counterparts. Because the grid-direct systems do not employ a method of storing energy, when the electric utility grid fails, they automatically shut down until the grid is back up. On the other hand, utility-interactive, backup systems, as shown in Fig. 2 (click here to see Fig. 2), do employ energy storage, typically in the form of a battery bank. These systems can operate specific loads that have been isolated from the main distribution panel and are connected to a backup load center. It is common for these systems to only run a few loads off the backup system, not the whole house. If a whole house backup is required, doing so with a battery bank and inverter system can be difficult.
All utility-interactive systems use a safety feature known as “anti-islanding” to prevent the solar array from remaining connected to the electric utility when the grid is down. The inverters used in utility interactive systems are required by UL1741 listing to disconnect themselves from the grid when the voltage or frequency of the electric utility falls out of a specific range. The inverters will automatically reconnect to the electric utility system only after the voltage and frequency values are within the acceptable range for five continuous minutes. This is a safety feature to protect electric utility line workers from being injured by PV systems that are pushing power back on to otherwise de-energized power lines.
Now that you have a better feel for the major types of systems being designed and installed today, let’s go over the major system components used in these systems. This list will not be all inclusive, but rather a 