On Grid Inverts or Grid-Tie Inverters

Grid-tie, or utility intertie, inverters convert DC power from Photovoltaic (PV) modules directly into AC power to be fed into the utility grid. Storage batteries are not needed, as any power produced that's not consumed by the owner's electrical loads is fed into the utility grid to be used elsewhere.

Most battery-less grid-tie inverters are "string" inverters, because the PV modules must be wired together in series to obtain a higher input voltage. String inverters are designed to run at voltages up to 600 VDC, which is the limit imposed by the National Electrical Code (NEC). String wiring is quick and easy to install and the higher voltage helps to minimize line losses and required wire size. However, in string wiring, maximum power point tracking (MPPT), along with any monitoring output, is performed at the string or array level. Central inverters are used in larger commercial grid-tie systems, typically more than 30 kW, as they are usually more economical than using multiple smaller inverters. Due to the high-voltages involved, string inverters should be installed and serviced only by qualified electricians.

Microinverters, such as the Enphase unit pictured here, are typically mounted behind each solar module. They convert the DC output of each module to AC, replacing the high DC voltages (up to 600 VDC) with comparatively lower AC potentials (120 to 240 VAC) and simplifying system design. The microinverter output connects directly to the breakers in the AC load center using conventional wiring. Since microinverters provide MPPT tracking and monitoring for individual modules, the impact of differences in orientation or shading between modules is reduced.

Module optimizers, such as those from Tigo and SolarEdge, mount behind each module like a microinverter and provide individual module MPPT tracking and monitoring, but have a DC output that is connected to a string inverter. These devices simplify system design and increase safety with a smaller financial and efficiency penalty than what is imposed by microinverters.

All grid-tie PV systems use the utility grid for energy storage. Whenever your PV array is generating more power than your loads are using, the excess energy is fed into the grid, literally turning the meter backward. When you need more power than the PV array can supply, the utility makes up the difference. Known as "net metering," this arrangement is the most efficient and cost-effective for grid-tied applications, since there are no batteries to maintain. However, most grid-tie inverters are required by law to shut down during a utility outage. If backup power is required, you will need a dual function grid interactive inverter.