buffalobill wrote:
"In a continuous UPS, the computer is always running off of battery
power and the battery is continuously being recharged. You could fairly
easily build a continuous UPS yourself with a largish battery charger,
a battery and a power inverter. The battery charger continuously
produces DC power, which the inverter continuously turns back into
120-volt AC power. If the power fails, the battery provides power to
the inverter. There is no switch-over time in a continuous UPS. This
setup provides a very stable source of power.

Standby UPS systems are far more common for home or small-business use
because they tend to cost about half as much as a continuous system.
Continuous systems provide extremely clean, stable power, so they tend
to be used in server rooms and mission critical applications. "
says
http://computer.howstuffworks.com/question28.htm

The small UPS systems (350 watts and up) are nice. They are intended to
keep your computer running only long enough to shutdown smootly,
typically a minute or two. Also, note the batteries in those units only
last about 2 years and replacements cost the same a replacing the whole
unit.