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. Sinewave inverters on the other hand, are more expensive as they require more components and are more complex to produce, particularly with dynamic loads. As such, sinewave systems are of higher quality, which is why we've identified them here. Sinewave systems provide maximum application compatibility as electrical devices are intended to be powered by a sinewave source.

The purity of the waveform is measured by Total Harmonic Distortion, or THD. This is the THD of the voltage (not current) waveform, and is therefore sometime referred to as THDV. (Current THD's are referred to as THDi, and this is load dependent, not UPS dependent when referring to the inverter characteristics).

Any waveform with a THDv of <8% is considered sinewave.

But that's not the whole story. Some inverters are better than others. This is determined by monitoring the quality of the waveform when subjected to different types of load.

For example, if you power a linear load from the sinewave inverter then you will normally find that the resultant waveform is also sinewave. (A linear load is one in which the current waveform is sinewave and in phase with the voltage waveform, such as is obtained with a purely resistive load, such as a filament light bulb).

If, however, we then take the same inverter and power a non-linear load, such as a rectifier, we may find that the waveform has distorted. In some circumstances the waveform may have distorted beyond 8% THD and is no longer considered sinewave.

A sinewave inverter that is sinewave for both linear and non linear loads is termed 'S'.

A sinewave inverter that is sinewave only for linear loads is termed 'X'