Grid-tie, or utility intertie, inverters convert DC power from PV modules into AC power to be fed into the utility grid. There are two major types of grid-tie inverters: string inverters and low voltage input inverters.
The SMA Sunny Boy, Fronius and Xantrex GT-3 inverters are string inverters. The name “string” comes from the way the way the PV modules are wired together in series to achieve a higher voltage. These inverters are designed to run at voltages up to 600 VDC. String wiring is faster to install, more efficient and allows the use of smaller gauge wire. DC voltage this high can be very dangerous and life-threatening, so string inverters should be installed and serviced by qualified electricians.
Microinverters, such as the one below, from Enphase, are bolted to the PV mounting structure beneath the solar modules. They convert the DC output of each module in a grid-tie system to AC, replacing the dangerously high DC voltage with comparatively lower AC potentials and greatly simplified system design.
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The micro inverter output connects directly to the breakers in the load center using conventional wiring. Microinverters provide MPPT tracking and monitoring for individual modules and allow modules to be installed in a wider variety of orientations and without the dramatic production losses caused by shading.
All grid-tie PV systems use the utility company, in effect, as a storage battery. When sun is shining, your electricity comes from the PV array, via the inverter. If the PV array is generating more power than you are using, the excess is sold to the power company through your electric meter; in such cases, your meter actually runs backward. When you need more power than the PV array can supply, the utility makes up the difference. There is no battery maintenance with this set up. Unfortunately, if the utility power goes down, this type of inverter will go off, too, regardless of whether or not the sun is shining.