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. High quality standalone inverters are available in sizes from 100 watts, for powering notebook computers and fax machines from your car, to 60 kilowatts, for powering a commercial operation.

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 PV modules are wired together in series to achieve a higher voltage. These inverters are designed to run at voltages up to 600 VDC.

The size of an inverter is measured by its maximum continuous output in watts. This rating must be larger than the total wattage of all of the AC loads you plan to run at one time. Wattage of most AC loads can be determined from a tag or label on the appliance, usually located near where the power cord enters, or from the owner’s manual. 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 for short periods of time while these motors start. Standalone inverters are available with two basic power output waveforms: sine wave, and modified sine wave (the proper term is actually modified square wave, but since modified sine wave is much more commonly used.)

Grid-tie inverters, dual-function inverters and utility companies deliver a sine wave. Exeltech, Xantrex XW Series, 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. Exeltech sine wave inverters are an excellent choice for power systems running audio or telecommunications equipment and other electronics that are waveform-sensitive. 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. We now carry Samlex sine wave PST inverters for a lower cost, smallsystem sine wave alternative.

Xantrex TR series, Magnum, and Samlex 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 high efficiency makes them economical with power when running small loads like a stereo or a small light. They can power most lighting, televisions, appliances and computers very well. Unfortunately, this type of inverter may destroy some rechargeable tools and flashlights, and laser printers and copiers. They may not allow many laser printers, copiers, light dimmers and some variable speed tools to operate. Equipment with silicon controlled rectifiers (SCRs) will not operate. Some audio equipment will have a background buzz that may be annoying to music connoisseurs.

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 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 voltages with comparatively lower AC potentials and a greatly simplified system design. The microinverter output connects directly to the breakers in the AC load center using conventional wiring. Micro-inverters 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.

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 at a cost slightly below that of a microinverter.

Central inverters are used in larger commercial grid-tie systems, usually of 50 kilowatts to 1 megawatt or more. They are the most economical way to convert the DC output of PV systems this large to AC for connection to the utility grid.

All grid-tie PV systems use the utility company, in effect, as a storage battery. When the 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. This type of system makes the most sense in most cases where you have utility power, because there are no batteries to maintain or replace. Unfortunately, if the utility power goes down, this type of inverter will go off, too, regardless of whether or not the sun is shining.