According to :
Errrr... Your having flashbacks to the old days...

Most modern UPS's either use a stepped wave (cheap ones)
or the more expensive units do make a sine wave. These
sine waves can be cleaner than what you get out of the
power grid.

Think about it... The square wave would be a problem with
transformers... In modern computers they ALL use transformers
to convert the power down to 5v and 12v.

Errrr..... Virtually ALL modern computers use switching power
supplies. Which is direct rectification of the AC to DC
(at 160V), filtering, and then AC modulation at some high
frequency (20Khz-60Khz or even higher) through a toroidal
step down transformer, then rectification back to DC, filtering
and regulation.

[The advantage to this is that the toroidal transformer
can be very small at this frequency. At 60hz, a "normal"
step down transformer at these power levels would weigh
20 pounds or more. In contrast, at 60Khz the toroid
weighs well under a pound. The economics are such
that these switching power supplies are vastly cheaper
to build and smaller than "classical transformer" supplies]

These power supplies are fairly insensitive to line quality,
and really don't mind square wave at all - in fact the high
voltage filtering is much more effective.

What these power supplies (and ordinary transformers) intensely
dislike is fast transients, or things that can cause them (eg:
_very_ sharply square square waves lead to inductive spiking
especially through ordinary transformers). These spikes
tend to blow holes through transformer insulation or blow
rectifiers and high-side capacitors.

"square sinewave" is an oxymoron ;-) The correct term
is "modified squarewave".
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.