Nominal Battery Voltage dictates the battery bank configuration. Only a few inverters (such as those from Exeltech) can accommodate multiple battery bank voltages. Most household-sized inverters require either 24 or 48 V battery banks. There are a few 12 V inverters on our list, but these are usually for smaller systems (think cabin-sized) that serve only a few AC loads.
AC Output Voltage has been limited to 120 VAC for a single inverter until the last few years. Now, several inverters have split-phase 120/240 VAC to power both standard 120 VAC loads and 240 VAC loads (such as a well pump). These inverters also have battery chargers to charge the battery bank using both legs of a 240 VAC generator. This saves generator run time and avoids having to include a 120/240 step-up/down autotransformer. Split-phase inverters also negate problems with wiring a single inverter to a load center that has multi-wire branch circuits, since this can possibly overload the neutral conductor. Additionally, several inverters on our list can also be connected in groups of three to supply three-phase 208 VAC output, commonly used in small commercial systems. (To see which models have this functionality, see the “Stackability” column in the table and look for “3Ph.”)
Peak Surge ratings reflect the inverter’s capability to supply significantly more than its continuous power rating for short periods of time. Certain appliances (i.e., those that have motors, like washing machines, refrigerators, and well pumps) will briefly draw more power upon initial startup. To find the surge requirement for a particular appliance, check the appliance spec sheet for the “start amps,” or contact the appliance manufacturer. Alternatively, you can measure it with a recording clamp-on ammeter.
Stackability is the capability to connect multiple inverters together to create 120/240 VAC output (series stacking) or increase output current (parallel stacking). Historically, the ability to series stack was handy for systems that needed to power 240 VAC loads, using inverters with 120 VAC-only output. Now that more split-phase inverters are available, stacking is usually done to increase inverter output capacity (amps). Stacked inverters can be programmed to activate only if needed so that when there is low power demand, standby losses are reduced (see “No-Load Draw”).
Peak Efficiency is the ratio of AC power out of the inverter to power in from the DC power source. The higher the efficiency, the less energy that is wasted in the inversion. Actual operating efficiency will vary depending on how much power is being pulled through the inverter, so inverter efficiency curves can be more helpful than the peak efficiency specification, and are often available in inverter manuals. On-grid systems spend most of their time processing RE-generated power to send to the home/grid, so high efficiency at the RE system’s power output rating is best. Since off-grid systems can spend much of their time requiring significantly lower power (when only a few loads are running), consider an inverter that has high efficiency at lower power output.
No-Load Draw (aka “Idle,” “Standby,” or “Tare” Loss) tells how many watts each inverter consumes simply by being “on.” This power needs to be accounted for when performing a load analysis for an off-grid system. Multiply the no-load draw by 24 hours to calculate the daily energy (watt-hours) consumed by the inverter.