Central or String Inverters are the most common, especially in residential solar electric systems. In cases where individual monitoring is required or shading and obstacles are present, using Microinverters is recommended. Central inverters with Battery Backup allow you to have a solar system connected to the grid and backup batteries to provide power when the grid fails.
- String Inverters
The solar panels in a grid-tie systems are connected to a Central or String Inverter to convert the voltage from the solar panels to a voltage compatible with the electric grid. A grid-tie string inverter must include the following characteristics for safe operation and comply with UL guidelines. The inverter must have a main switch to completely shutdown the entire system, it must have a Ground Fault Detection and Interruption (GFDI) device and an Arc Fault Detection and Interruption (AFDI) device. The inverter must be shut off for 5 minutes after the voltage in the utility grid drops. String inverters can be ordered in 240V, 208V and 277V configurations; some units are field configurable adjusting to single phase and three-phase systems. All central inverters feature a display that will show various production values such as daily generation, cumulative generation, current voltage, etc. Some inverters feature a series of LED lights in the front face that will alert the user when something is not working and needs attention from a technician. The inverter is the device that decides the main functions of the system such as when to start operations in the morning. It is very important to select the right inverter in order to ensure the best performance of the system during its 30-year life cycle. All inverters are designed for outdoor use; however, they should always be installed in a cool and well ventilated location and never in direct sunlight.
The use of microinverters allows for more advanced monitoring capabilities as well as for a more flexible installation. As opposed to string inverters, you will have one microinverter per solar panel and each pair of solar panel+microinverter is independent from the rest of the system. Microinverters pose an unique advantage when there is shading from nearby objects like trees and buildings. Microinverters can be found in 240V configuration for single phase systems and 208V for three-phase installations. Similar to central inverters, they comply with all UL safety regulations. Microinverters do not have moving parts, switches or buttons. They are outdoor rated and are usually mounted under the solar panels. They do not feature independent displays; however, the system needs to be supplied with a display monitor that will allow you to keep track of your systems performance. The monitor will also allow you to check the systems performance remotely via the internet. One feature that is only available with microinverters is the capability to monitor the performance of each panel individually. The visual interface makes it very easy to identify when a solar panel has been disconnected or is not performing at its optimum capacity. Microinverters are also recommended when the solar panels are installed in challenging locations, for example a roof top with skylights, chimneys, vents, pipes, antennas, etc. In these types of scenarios, the solar array needs to be broken down into smaller sub arrays, a task that is simplified by using microinverters. The smallest string inverter available is 2000W, if a smaller grid-tie system is needed, microinverters are the best option.