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ITGlossary · UPS technology
Pure Sine Wave
UPS output with the same waveform as the mains, needed for PFC power supplies, motors and sensitive equipment, unlike a stepped approximation.
Technical level:Intermediate
In practice the output voltage follows a smooth sinusoidal curve, just like the mains, without the steps of an approximated waveform. The difference matters mainly when the UPS runs on battery; online UPSs deliver a sine wave at all times.
Many modern computer and server power supplies use active PFC and, on a stepped waveform, may hum, run hot or shut down at the moment of transfer. Motors, pumps and transformers also heat up more. Quality is measured by total harmonic distortion (THD), which on a good online UPS with a linear load is usually below 3%.
Before buying, check the datasheet for the waveform in battery mode; terms such as “simulated sine” or “stepped” do not mean a pure sine wave.
Questions that mention it
All frequently asked questionsWhat is a sine wave output and why does it matter?
A pure sine wave output closely matches the quality of utility power. It is essential for sensitive devices such as medical or telecommunications equipment.
Related terms
- Harmonic DistortionDeviation of the waveform from a pure sine wave, caused mainly by non-linear loads such as power supplies and inverters.
- AC EfficiencyIndicates how efficiently a UPS converts incoming AC power into usable output power during normal operation.
- DC EfficiencyMeasures how effectively a UPS converts battery power into output power during battery mode.
- Double Conversion EfficiencyThe share of input power that an online UPS delivers to its output, lower at light load and highest at medium to high load.
- Maintenance BypassA manual switch used to isolate the UPS for servicing while still supplying power to the load.
- Offline (Standby) UPSA UPS that leaves the load on the mains and switches to battery only when the mains fails, with a short transfer time; the simplest and cheapest topology.