UPS (Uninterruptable Power Supplies)... kinda long.

[Phil R. Karn]

The reason there are at least three different UPS/SPS configurations shows
that there is no one ideal approach. The SPS (standby inverter plus
relay) has the advantage of low cost and efficient operation, but at
the expense of a switchover transient when the power fails. The SPS-plus-
ferroresonant transformer (the BEST Technology approach) has the advantage
of covering the switchover transient and of providing something that looks
more like a sine wave, but at the expense of weight, accoustic noise
and inefficiency. The true online UPS has the advantage of an output
that never wavers across an outage, but at greater expense (since the
inverter must be rated to run continuously) and lower efficiency (because
in normal operation, power is being double-converted).

In general, the switching power supplies used in most computers can
handle the switchover transients of a SPS just fine. Linear power
supplies, however, can have problems because their low voltage filter
caps can't store nearly as much energy as the high voltage caps found in
most off-line switching supplies. This problem bit me in our Internet
gateway; we have a Cisco CGS router plus a separate T-1 CSU, both
powered by an Inmac 400VA SPS (OEM'ed from American Power Conversion).
Although the Cisco would ride just fine across the switchover transient,
the CSU occasionally glitched and hung, and it had to be power cycled to
get it going again.  My solution was to install a ferroresonant
transformer between the SPS and the CSU.

So, in general, SPS's are just fine for most computer equipment. And if
you have more sensitive loads (modems, etc), add a small ferroresonant
transformer to protect them.

Phil