Most modern off-grid PV systems use sine-wave inverters. But plenty of older modified-square-wave inverters—what are still euphemistically marketed as “modified-sine-wave” inverters—remain in use today. The most common are the Xantrex (Trace) DR and earlier U-series, and most RV and marine inverters.
These inverters have special generator requirements. While modern inverters use the entire power waveform (the technical term is “power-factor-corrected”), older modified-square-wave inverters use only the voltage peak of each wave. What this means is that the generator must maintain full peak voltage under a charging load.
High-quality generators are able to maintain peak voltage (the ideal is 164 to 170 V on a 120 V wave) under load. But cheaper generators tend to have a flat voltage waveform —that is, the measured RMS voltage may be accurate, but the peak is lower than 164 V under load. This is adequate for occasional power tool use, but can result in very low charging current—the familiar mystery of low-DC charging current from a well-running generator.
Three possible solutions address this problem. The first is to oversize the generator, so that it is lightly loaded while charging. Second is to increase engine speed, which works in some generator-inverter combinations by increasing voltage proportionally. The best solution is to use a modern inverter–generator. These units maintain full peak voltage up through their maximum load and, if sized correctly, will enable a modified-square-wave inverter to charge at its full capacity. Most inverter–generators have 120 VAC output, which is ideal for a single-inverter power system.