Whistler XP100i Owner's Manual page 14

OPERATING PRINCIPLES
The majority of AC voltmeters measure RMS voltage and assume that
the measured waveform will be a pure sine wave.
Consequently, these meters will not read the RMS modified sine
wave voltage correctly and, when measuring your WHISTLER inverter
output, the meters will read about 20 to 30 volts too low. To
accurately measure the output voltage of your inverter, use a true
RMS reading voltmeter such as a Fluke 87, Fluke 8060A, Beckman
4410, Triplett 4200 or any multimeter identified as "True RMS."
For more information on inverters see our Inverter FAQ page.
A true sine wave typical of home AC outlet.
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