Brodric is giving good advice.
You need to read the fine print on UPS’s. As Brodric says, cheap ones give lots of distortion. Some that claim things like “Really good Sine” (not really but it makes the point) may have a whole three levels instead of two 
Look for the distortion from a mathematics sine (e.g. THD, or number of steps or whatever) to find the better ones. The big problem is that a good UPS with a good sine wave output is either very expensive or it isn’t very efficient. Low efficiency means that means it’s hot which means that it needs a fan… Check the noise level in (in some form of dB.) If you don’t get a double conversion you’ll need to make sure that the time to switch to backup on a black or brown out is short enough that the equipment down stream can idle thru… That’s hard for units that use a lot of current (e.g. an amp or a power conditioner.)