Photovoltaic solar systems generate DC voltage, and an inverter converts the power to AC voltage. Solar inverters produce a sine wave and are designed for high power—up to hundreds of kilowatts. Unlike simple electronics inverters, solar inverters provide numerous functions in addition to DC-to-AC conversion. They are responsible for energy metering, monitoring, regulation and protection of the solar energy system.
Solar energy systems can be designed as on-grid or off-grid (isolated) systems. Off-grid systems are designed to work independent of the electrical network, while on-grid systems can supply energy to the network. On-grid systems can be set up with or without a battery storage system, which can be used for backup power. Inverters are a mandatory component in both types of systems.
Energy in a solar energy system can flow in different directions. In the case of a simple system in a home application, the users can be supplied only from PV panels. If there is insufficient sunlight, the users can be supplied from both PV panels and batteries, or only from the batteries (e.g., during the night).
A very important function of solar inverters is their role as the solar charge controller. Depending on the energy state of the PV panels, the solar inverter will direct energy to users, battery charging or the network. The charge controller connects the PV panels, batteries and users, but also protects the battery from overvoltage and deep discharging. The battery voltage is adjusted automatically depending on the battery type, condition and temperature, while the user supply voltage must be constant.
Another important task of the solar inverter is providing protection and safety. This is especially important in on-grid systems, as there are strict regulations when a solar system is connected to the network. In this case, the inverter must synchronize the voltage and frequency of the solar plant to the network. It also must synchronize any disconnections from the network during outages, as it can be problematic if the solar system continues to supply energy even though the power grid is no longer in place. This is called islanding, and it can be dangerous to workers who may come in contact with powered circuits. Solar inverters should be able to detect islanding and disconnect the solar system from the circuit—a feature called anti-islanding.