Time needed for a UPS to switch from line power to battery?

"Depends on the type of UPS. From Electropaedia:

Off Line or Standby UPS

These are simple inexpensive systems
providing only basic protection. In
normal situations the UPS passes the
mains power directly to the load. The
mains power provides a single DC line
which keeps the battery charged. When
the UPS detects a voltage too low, it
turns on the inverter to power the
load from the battery. The system is
relatively slow (longer than 4 ms) and
the delay between mains power loss and
inverter startup can be long enough to
disrupt the operation of some
sensitive loads. This technology does
not normally provide full time power
conditioning but it may use a simple
filter to clip spikes and electrical
noise.

On Line or No Break UPS

These are designed to provide a zero
transfer time, with better voltage and
frequency regulation than that can be
achieved by off line and line
interactive UPS. In on line systems
the mains power is used to provide two
DC power lines which feed both the
charger and the inverter which is
permanently turned on providing the AC
power to the application. When the
mains fails, the inverter
instantaneously draws its DC supply
from the battery instead of the mains.

The method by which the AC load is
permanently supplied by the system
inverter and not the mains is called
""double conversion"" since the charger
provides the AC-DC conversion and the
inverter converts the DC back to AC
again.

On line systems typically provide full
power conditioning, protecting the
load from all forms of power
disturbances, including brownouts,
blackout, transient surges or sags. In
the event of a mains power failure,
there is no delay or transfer time to
backup power. These systems are
however more costly and have both
higher power consumption, and higher
heat generation.

Line Interactive UPS

These systems contain an off-line
inverter but also use a transformer to
supply to the load. In the event of a
mains failure the inverter is started
and switched to the transformer to
provide the output. The transformer is
used to provide line conditioning
however it also maintains output on
its secondary briefly when a total
outage occurs increasing the hold up
time of the UPS. This results in a
break in the output of a few
milliseconds or less and is thus
faster than a simple off line UPS.

In my office, we have several of the ES style APC UPS's. Our building has a diesel-powered generator that comes on within 1-2 seconds after a power loss, and we use these UPS's to cover the period between power loss and the generator turning on. I've gone through their documentation for these models and I can't figure out what type they are. I also use these same UPS's at home and my computers have stayed on during a power loss."