String inverters interface a single PV string to the grid. They can be subdivided into single and two stage conversion topologies, depending on the addition (or not) of a dc–dc stage used to adapt the dc voltage output from the PV string to the dc side volt- age of the grid inverter. In addition, the dc–dc stage decouples the MPPT control from the grid side control (active and reactive power) by enabling a fixed voltage at the inverter dc side. Furthermore, grid inverters can be found with or without galvanic isolation. Isolation can be introduced at the grid side with low-frequency trans- formers (large and heavy) or within the dc–dc stage with a high-frequency transformer (light and compact but with additional losses from several dc–dc converter semiconductors). The different combinations between single or two stage, with transformer or transformerless string inverters, has led to a wide range of different configurations as shown in Figure 3. Compared to ac-module inverters, the string inverter has a less accurate MPPT of the PV systems and, under partial shading, would reduce the energy yield. However, for a PV system of the same power rating, the string inverter has a lower cost per watt and is more efficient. The string inverter is very popular for small to mediumscale PV systems, particularly for residential rooftop PV plants.