Many off-grid inverters can operate in parallel with a generator, instead of just switching the loads to generator power when the generator comes on. This allows an inverter to “assist” a small generator with large loads. Historically, generators were sized to simultaneously power the largest loads and charge the batteries. Now, with greater inverter capacity, the inverter may be sized to serve the largest loads, with a small generator sized to handle only battery charging. The inverters that can operate in parallel with a generator (often called “generator support”) can help a smaller generator start a large load like a well pump or table saw by briefly drawing power from the batteries.
Some inverters provide only 120 VAC output; if your off-grid house (or the critical loads subpanel in your battery backup system) requires 240 volts, a second inverter is added to provide the other phase. Instead of adding a second inverter, an external step-up transformer can be used to get 240 VAC from a 120 VAC inverter. The efficiency is reduced, but the cost may be quite a bit lower than adding a second inverter. Some inverters come with 120/240 VAC split-phase output. Which is best depends on your situation.
If you have no 240 VAC loads, you can use a single 120 VAC inverter to energize both 120 VAC legs of your load panel (see the “Beware: Multiwire Branch Circuits” sidebar). If you have an appliance that requires 240 VAC, such as an existing well pump, you have a couple of choices:
A 120/240 VAC inverter is often selected for a battery-backup grid-tied system because it’s cheaper and easier to install. The amperage of the tie-in is half as much at 240 V compared to the tie-in at 120 V. This means you can fit twice as much PV power on a given service size following NEC 705.12(D), which commonly limits the size of the solar input to 20% of the busbar amperage.
Remember that an inverter is only one part of the system—many people focus on selecting and buying the inverter, and then face the challenge of integrating it with rest of the equipment. Magnum Energy, OutBack Power, and Schneider Electric offer wiring solutions (aka “power centers” or “power panels”) for use with their inverters, simplify the wiring considerably. There are also third-party options, such as MidNite Solar’s E-Panel, which provide complete Code-compliant wiring systems to simplify an inverter’s installation. Most inverters from Magnum Energy, OutBack Power, and Schneider Electric require a separate system control panel for programming and monitoring. There are no controls or displays on the inverter itself. This can be good when the inverter is located in a utility room, but, for example, you also want an inverter control/monitor in the living room. This functionality comes at an extra cost—between $150 and $400 depending on the model.
Many of the advanced functions, such as automatic generator-start, are part of the system control panel, not the inverter firmware—without the control panel, you may be limited to just turning the inverter on and off, and not be able to adjust the settings.
Many battery-based inverters can connect to a computer for remote monitoring, control, and data logging. Some allow users to remotely monitor the inverter’s operation via the Web. This usually requires an extra communications box (which may or may not be the same as the remote system control panel).