William Sommerwerck wrote:
Some UPSs actually power the load continuously and are
recharging the battery from the AC line (like telco's operate).

Actually, /all/ UPSs work that way. The common type of unit -- which costs
less -- is properly called a Standby Power Supply (SPS). The circuitry
doesn't come on until power is lost.

Sorry, I wasn't clear:

"THE INVERTERS IN some UPS's actually power the load continuously
and are recharging the battery from the AC line (like telco's
operate)."

This is typically called an "online" UPS. Net current flowing
into/out of the battery is zero once charged. But, current
flowing into the *inverter* is directly proportional to the
load current being supplied EVEN WHEN AC POWER IS AVAILABLE.
I.e., these have zero transfer times. This is how the telco
CO's operate (in the US, at least) -- everything runs off battery
though those batteries are continuously being charged (replenished)

Cheaper UPS's, by comparison, are "offline" UPS's -- the
battery is charged from the mains "as needed" but the inverter
only supplies power to the load when AC power has failed.
Typically, a switch disconnects the load from the AC mains
and connects it to the inverter's output in this situation.
As such, there is a nonzero transfer time as the switch
flips from one "position" to the other. (note that this
switch is not present in the online UPS!)

Online UPS's provide isolation of the load from the mains.
As such, fluctuations in the mains (phase and or magnitude)
are insignificant (except to the extent that they hinder
charging of the battery). OTOH, the electronics are continually
stressed in these UPS's *and* the user sees an apparent increase
in power consumption as the inverter's (in)efficiency is *always*
reflected in the power drawn from the mains.

Loads serviced by offline UPS's are exposed to the mains
until the UPS decides otherwise. Since the UPS can only
detect problems with the mains after the fact, the only
remedy that the UPS has to a detected problem is to switch
the load to the inverter's output to bridge the "problem".
As such, it is impractical to handle cycle-at-a-time
problems on the mains.

In these scenarios, "line interactive" UPS's can be a win as
they allow the UPS's output to be adjusted *without* the
use of the inverter -- typically, by dynamically switching
the taps being used on an autotransformer in series between
the mains and the load (i.e., this only applies to operation
on the mains). For small-ish loads, these usually aren't
worth the added cost/weight (as most electronic devices
can usually adapt themselves to a wide range of mains
voltages).