As stated previously, a UPS is intended, in its simplest form, to provide backup power when the incoming mains fails. The power waveform that is produced is generated by a circuit known as an inverter. The quality of this inverter has a major impact in the quality of the UPS as a power protection solution. It may be expected that the UPS would deliver a waveform that is equivalent if not better than, the waveform that is normally present. Unfortunately this is not always the case. In order to save cost, many UPS systems operate with non-sinewave inverters as this is a simple and cheap way of generating an AC power waveform. Some manufacturers try to mask the output waveform by referring to such systems as simulated sine wave or pseudo sinewave. There may be some improvements over a pure square wave, however regardless of this, the fact is, they all have high harmonic content and are therefore suitable only for specific types of load. Squarewave systems have their place but one should understand the limitations of such devices. The square wave nature means that they are sources of extreme electrical noise causing disruption, degradation and damage to electronic circuits. They are not suitable to power any system that contains coils, inductors, transformers or requires a stable noise floor to work effectively. They are suitable as a backup power source in the event of infrequent power cuts for non-essential equipment.

Any waveform with a Total Harmonic Distortion of >8% is considered non-sinewave, which is why pseudo-sinewave or stepped sinewave are nonsensical terms. Square wave UPS systems can have a THDv of upto 80%

A square wave inverter is given the classification 'Y'