Well, the peak to peak voltage of a 120 VAC RMS (typical house voltage, in other words), is 120 * 1.41 * 2 = 340(ish), so the peak voltage is going to be half that, or 170 volts. We call it 120 volts because that is the RMS value (root of the mean of the squares of the sine wave, or .707 times the peak voltage) which is the DC equivalent of the AC waveform. Aside - cheaper AC voltmeters read in pure sine wave RMS, so if you test your house voltage you will get a reading of 120 VAC instead of 170 or 340. Expensive "true RMS" voltmeters measure the actual waveform presented and calculate the actual RMS voltage of the waveform, which doesn't have to be a sine wave. That is why measuring the output of a MSW inverter can present some funky readings with normal voltmeters.

It sounds like your inverter needed a power-off restart. I suspect that could have been due to some glitch or brown-out on the input from your AC source, or some more nefarious problem. You're just going to have to watch it for a while.

Brian