For accurate measurement of the output voltage of the inverter, a voltmeter with "True RMS Voltmeter" marking on it must be used. Typical brands of this voltmeter are; Fluke 87 and 8060A, Beckman 4410 or Triplett 4200. If your voltmeter is not one of these brands or does not have a "True RMS" marking on the unit, it will read 20 to 30 volts low.
The modified sine wave output confuses non true RMS meters.