Efficiency Ratings. Manufacturers have their own efficiency ratings, but for accurate production projections, it’s wise to obtain them from an independent source, such as the Go Solar California Initiative (gosolarcalifornia.org). Manufacturers’ efficiency ratings (often labeled “Maximum Efficiency” on spec sheets) could be based on testing at lower ambient temperatures and under other conditions that do not reflect realistic field operation, but optimize inverter performance. The Go Solar California Initiative ratings (labeled “CEC Efficiency” on spec sheets) will be slightly lower (generally around 1%).
An off-grid inverter is sized differently than a grid-tied inverter—the latter has grid power available to supplement PV production. An off-grid inverter must have enough AC output power to cover any AC loads that might run simultaneously. For example, if you have 2,850 W of AC loads, then you might choose an inverter with 3,000 W of continuous output power. Any surge requirements (common with compressors and motors, such as in refrigerators, well pumps, etc.) need to be checked against inverter “surge” or “overload” specs (see “Sizing a Battery Based Inverter” in HP149).
Since the inverter draws from a battery bank instead of the PV array, the DC voltage input needs to match the nominal battery bank voltage (generally 12, 24, or 48 V). Inverter model numbers usually reflect both the AC power output and the DC battery voltage. For example, the OutBack VFX 3648 can produce up to 3,600 AC watts continuously and works on a 48 VDC battery bank.
Most quality stand-alone inverters produce a pure sine wave. The AC electricity from the utility is a fairly clean sine wave form, and higher-end inverters match or beat this quality. Modified square-wave inverters tend to be less expensive, but the power quality is not as good. Many appliances actually run fine on a modified square wave, but some loads, including thermostats, clocks, fans, and power tool battery chargers, might not function properly. Sine wave inverters are highly preferred, as they will run motors more efficiently, and consumer electronics with less noise and electrical interference.
AC Output Voltage. Some off-grid inverters have a 120 VAC output, requiring “stacking” two inverters to make 240 V, or need a 120/240 step-up transformer. Other inverters have single-phase 120/240 V AC output. Since most larger generators are also 120/240 V, they should be connected to an inverter that accepts a 240 V AC input, or run through a transformer to step it down to 120 V. Using only one 120 V leg of a 240 V generator can increase generator run time. Three-phase AC loads can also be run with some inverters. Each manufacturer will specify if this is possible and how it is done.
Other Options. Off-grid inverters have many helpful features and programming options. Most have built-in chargers to charge batteries from an AC power source. They should also have a programmable low-voltage disconnect (LVD) to keep batteries from being overdischarged. Other options may include automatic generator starting, battery monitoring, remote displays, a Web interface, and auxiliary relays (for various functions such as operating a battery box fan).
A grid-tied with battery backup (GTBB) inverter has to work both as a grid-tied inverter and a stand-alone inverter to energize loads when the grid is down. Choosing is fairly simple, since there aren’t very many available models (see “Choosing a Battery-Based Inverter” in HP149).
GTBB inverters must have a large enough AC power rating to pass through the full PV array output (like a grid-tied inverter) and to power the backed-up AC loads subpanel (like an off-grid inverter). For example, a 4 kW PV array with 2.5 kW of loads on the backed-up subpanel necessitates using a 4 kW inverter to meet the PV array requirement even though the load power needs are less (surge load sizing applies, as well). The battery bank voltage must match the inverter, as in off-grid systems, and the AC output voltage must also match the utility voltage, as in grid-tied systems.