An inverter’s job is straightforward—change PV-generated or battery-stored DC electricity into AC electricity that can be used by household appliances or sent back to the utility grid. However, with a plethora of models and options, shopping for an inverter has become a more complex task. With some basic know-how, you can narrow down the choices to select the right one for your application.
The most basic choices are whether your system will be grid-tied or off-grid, and whether your system will include batteries, either for off-grid use or on-grid with battery backup. Inverters are specific to each.
Grid-tied systems are the most common PV systems in the United States. Most do not use batteries; the energy from the PV array is sent straight through the inverter to the AC loads or the utility grid. There are more brands and models of batteryless grid-tied inverters on the market than any other inverter type.
When there are batteries in a PV system, an entirely different type of inverter must be used. Off-grid (stand-alone) systems use batteries for energy storage. Inverters for these can be divided into two categories, based on some of their features. Larger battery-based inverters have a wide range of programmable features and options, and are designed to run a whole house’s loads, as well as regulate and charge the battery bank from an AC power source. Smaller battery-based inverters tend to be much simpler, don’t include AC battery chargers or battery monitoring, and are used predominantly in applications where there are only a few AC loads.
Grid-tied systems with battery backup can send excess energy to the grid, but when the grid is unavailable, will still energize loads using energy stored in the batteries. While many battery backup grid-tied inverters can be used for off-grid applications, the inverse is not the case—most off-grid inverters cannot interconnect with the grid.
AC Output Power. First, match the inverter’s AC output power to your PV array. A PV array’s standard test conditions (STC) power rating will give you a good idea of the minimum size of inverter needed. For example, a 5 kW PV array will need an inverter with about a 5 kW power output. If the climate is such that the array never reaches STC power—such as in warmer sea-level climates—you may be able to “undersize” your inverter by 10% to 20%. In our example, undersizing would call for a 4 to 4.5 kW inverter for that 5 kW PV array. Arrays in colder, sunnier areas, such as at higher elevations, may necessitate “oversizing” the inverter to avoid power clipping—when the inverter cuts off some of the PV power because it can’t produce more than its rated power. There’s no danger in choosing an inverter with a larger power output, and the price difference for a slightly larger inverter is usually small. Additionally, a larger inverter can capture intermittent moments of high PV output, such as those stemming from cold, sunny days or from “edge-of-cloud” effect—and possibly leave room for future expansion (see “Ask the Experts: Sizing Inverters for Cloud-Edge Enhancement” in HP143).
DC Voltage Input Window. This is the voltage range that the inverter will accept from the PV array. Input voltage for U.S. residential inverters will never be above 600 VDC, and some models will have considerably lower values. Since cold temperatures increase PV array voltage, and high temperatures decrease it, temperature coefficients and local climate data will determine the highest and lowest expected voltages. Exceeding an inverter’s maximum voltage specification may cause inverter damage or worse (fire), and while not dangerous, array voltage that’s too low can cause the inverter to stop processing power.
Most grid-tied inverter manufacturers offer online string-sizing programs that allow you to select specific PV modules, and input minimum and maximum ambient temperatures. The program will display how many modules can be connected in series to fit a particular inverter’s DC voltage window (see the “Online String Sizing” sidebar).