Depending on the Genrator (Traditional or inverter)
In "Traditional" Generators there are devices that are designed to control the wave form and "Total harmonic Distortion"
Sitting in the box these devices get... Weak.
HOWEVER DO NOT WORRY
First. Resistive stuff like lights. could care less about the waveform.
And second. Well the one i bought where that happened by the 2nd tank of gasoline was 100%. The devices had "Woke Up" and were working.
(Actually I'm blabering. i do know the proper technical terms but .. Well.. They do not matter).