When measuring output with a voltmeter, why does my inverter read 20 to 30 volts lower than stated on the box?

The inverters produce modified sine wave with a root mean square (RMS) voltage of 115 volts, the same as standard household power.

Most AC voltmeters (both digital and analog) are sensitive to the average value of the waveform rather than the RMS value. They are calibrated for RMS voltage under the assumption the waveform measured will be a pure sine wave. These meters will not read the RMS voltage of a modified sine wave correctly, and instead report the sine wave to be about 20 to 30 volts lower than it is, when measuring inverter output.

For accurate measurement of inverter output, you should use a true RMS-reading voltmeter.