Measuring Voltage of Modified Sine Wave Inverters
by Windy Dankoff

Refer to article: What is a Sine Wave?

If an an inverter is not a true sine wave type (a so-called "modified sine wave" inverter), a simple voltmeter or multimeter will not read accurately. Typically it will show around 95-105V. However, the normal amount of power WILL be transfered to the load. This DOES NOT indicate a problem, but we frequently hear from customers and electricians who notice this discrepency.

In order to measure a non-sine wave or a distorted waveform, a more expensive "true RMS" meter is required. (RMS is root-mean-square, a sort of exponential averaging.) Prices of RMS multitestors begin at around $100 in USA. If you don't have one and you want to confirm proper inverter output, simply connect a lamp with an incandescent light bulb. If it looks like its normal bright white, running on any modern inverter, you can safely assume that the RMS voltage is in its normal range.