Whistler XP200i Owner's Manual page 18

OPERATING PRINCIPLES
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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