I've used 120V input VFDs up to 0.4KW with no problem. The problem would start with higher power ratings. A 1 KW motor at 120V is going to draw around 15 amps which is going to max out many 120V circuits. A 4 KW motor is going to draw close to 50 amps on a 120 V circuit. Putting the VFD between the wall and the motor does not change the amp draw at the wall, except to increase it a bit.