Off-grid, or standalone, inverters convert DC power stored in batteries to AC power that can be used as needed. Select an inverter for your power system based on the maximum load you will be powering, the maximum surge required, AC output voltage required, input battery voltage and optional features needed.
If the inverter is expected to run induction motors, like the ones found in top loading washers, dryers, dishwashers and large power tools, it must be designed to surge, or deliver power many times its rating fro short periods of time while these motors start.
Standalone (Off-Grid) inverters are available with two basic power output waveforms: sine wave, and modified sine wave. Exeltech, Xantrex XW, SMA Sunny Island, Magnum MS and OutBack FX inverters are sine wave off-grid inverters. Sine wave inverters have a higher cost, but they can operate almost anything that can be operated on utility power. The OutBack and Xantrex XW series inverters can be ganged together for up to 36 KW of output and can operate off-grid or grid-tie.
Xantrex TR series, Magnum, and Samplex PSE inverters have modified sine wave output with harmonic distortion of around 40%. They are an economical choice in power systems where waveform is not critical. Their high surge capacity allows them to start large motors while their efficiency makes them economical with power when running small loads like a stereo or a small light. Unfortunately, this type of inverter may destroy some rechargeable tools and flashlights, and laser printers and copiers. Equipment with silicon controlled rectifiers (SCRs) will not operate. Some audio equipment will have a background buzz that may be annoying to music connoisseurs.
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 at the right, 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.
The microinverter output connects directlyto the breakers in the AC 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.