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Does the sine wave function of a UPS matter even when there is power from the grid?

I'm considering getting a UPS for my home studio computer (recent Mac Mini), router, sound card, speakers, and analog synths.

The purpose is just to save the equipment in case of a power outage, and give me enough time to shut everything down. The purpose is not to be able to keep on working with the equipment when there is no power.

I am considering getting a "sine wave" (i.e. non-stepped sine wave) UPS (perhaps something like the CyberPower ones, although I read that they may not provide a "pure sine wave.")

My understanding is that stepped sine waves may interact negatively with audio equipment and some computers. Since the equipment will constantly be plugged in behind the UPS regardless if there is power or not, I'm trying to understand to which degree a "sine wave" UPS is really necessary.

Does the UPS "sine wave" function matter ALL the time regardless if there is power or not, because the equipment is constantly plugged in behind the UPS, so not having UPS “sine wave” could/would disturb the audio quality even under normal operations with power from the electricity grid? In that case, "sine wave" seems virtually necessary to have.

Alternatively, does the UPS “sine wave” function matter ONLY when there is a power outage, in order to maintain close to normal functioning of the equipment? In that case, I imagine I can do without "sine wave" because I will not be using the audio equipment while on battery power.

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    \$\begingroup\$ You could not expect to get a pure sine wave from a power grid equipped with switched mode solar panels inverters, high-voltage direct current (HVDC) electric power transmission, thyristor controlled loads and large DC loads via bridge rectifiers. There will be always some harmonics, with or without the UPS. Producing a pure analog sinus with a UPS will have a bad efficiency. \$\endgroup\$
    – Uwe
    May 22 at 15:39
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    \$\begingroup\$ If the UPS is only to provide power for a leisurely shutdown in the event of grid failure, then your waveform quality doesn't matter when the grid has failed. You don't get a good sinewave out of the grid anyway. Some audio products with poor input susceptibility will be more disturbed by stepped waveforms than by better-filtered ones. Putting mains input filters on them may be more effective than the 'right type of UPS waveform'. Some 'instant switchover' UPS operate all the time, some don't operate until the grid fails. If you can 'try before you buy', then so much the better. \$\endgroup\$
    – Neil_UK
    May 22 at 15:47
  • \$\begingroup\$ Neil_UK's suggestion of "try before you buy" is also a good idea in case you get a unit that makes audible noise. \$\endgroup\$ May 22 at 16:16
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    \$\begingroup\$ It depends on the UPS type. There are different types. Some bypass the input mains to output if it is present, some always generate the mains output and just uses the mains input as power supply. Read the manual. \$\endgroup\$
    – Justme
    May 22 at 16:38

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For your specific use case waveform quality would only be an issue with an “online” or “double conversion” UPS. These types are constantly running their inverters to power the load and if the waveform quality is poor you will have constant issues related to that. Also note that this type of UPS is usually much more expensive than the following type.

Other UPS’ only activate their inverters when there’s a grid fault, so you would not be suffering from poor waveform quality most of the time. These types are called “line interactive” or “backup” or “standby”

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