The largest recent shift in inverter technology is the availability of transformerless inverters in the United States. They have long been popular in Europe, but now most inverter manufacturers have added a transformerless option to their existing inverter line. Without a heavy transformer, they weigh about 50% to 70% less than a transformer-based inverter of similar output, and the size of the inverter housing can be (but isn’t always) reduced. Inverter efficiency is also increased—there are no longer losses associated with having a transformer to step up the voltage. And because the transformer (which is comprised of copper windings on an iron or steel core) is eliminated, they are less expensive to produce.
The majority of inverter manufacturers are now including a transformerless inverter line. Exeltech, Ingeteam, and SolarEdge are a few examples.
However, transformerless inverters are not without some drawbacks. They require the DC wiring to be ungrounded. Because neither the positive nor negative conductor is connected to ground, they must meet more NEC requirements, per NEC 690.35, including the use of PV wire (a double-insulated single conductor cable having added sunlight and mechanical protection) for exposed wires (i.e., module interconnects and exposed home run wiring). Overcurrent protection and disconnect devices are required on both the positive and negative conductors, since they are both ungrounded. Arrays that require the positive conductor be grounded (those using SunPower modules, for example) are not recommended for use with transformerless inverters, because the array must be ungrounded.
Some installers contend that ungrounded systems are inherently safer, since the equipment-grounding conductor (which is connected to all metallic enclosures, the module frames, and racks) is no longer connected to a current-carrying conductor (commonly the negative). This reduces short-circuit potential should the positive current-carrying conductor come into contact with grounded metallic components. (For example, during a system checkup of a grounded system, if your screwdriver inadvertently comes in contact with a positive terminal and grounded metal in an enclosure, this short-circuits the positive and negative conductors.)