Except for instantaneous measurements, when we talk about our A.C. voltage, we are usually referring to the rms (root mean square) voltage. There are some volt meters (called true rms meters) thar actuall measure the rms voltage. They tend to be quite expensive.
Most meters actually measure the average voltage, and then apply a conversion factor to display the rms voltage. The conversion factoe they use is based on a true sine wave. A modified sine wave requires a different conversion factor. Unfortunately, your meter does not know that it is measuring a modified sine wave, and wouldn't have the correct factor, even if it did know.
Consequently, the value that it is displaying will be in error, by a considerable, but unknown amount. Any reading from a MSW inverter should be considered to be a very rough approximation.
Retired electronics engineer. Avid paddler & birder.
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