Cheap UPS inverters can actually have much higher harmonic distortion than mains power. For laughs, try plugging in a UPS with power quality detection into anot

Cheap UPS inverters can actually have much higher harmonic distortion than mains power. For laughs, try plugging in a UPS with power quality detection into another UPS as its "mains" source, it will probably detect the other UPS as providing poor quality (i.e. high THD) power. If you don't actually need UPS, it's probably easier to use an AC low-pass filter to remove harmonics than back-to-back DC conversion.