Electronics Communications Question:

While both provide backup power during mains outage, with
the UPS the switch is instantaneous whereas with the
Inverter there is a gap of a second or two. This gap is OK
for household gadgets such as lights, fans, fridge, etc. but
not OK for computers.

In technical terms:

UPS: The mains power comes to the UPS. The AC is converted
to DC and this DC is constantly charging the battery. The
output of the battery is fed to the Sine wave inverter and
it converts DC to AC and this feeds the equipment. Since
power out is always drawn from the battery, there is no time
lag when mains swicthes off; it justs stops the battery from
being charged and the UPS continues to supply power till the
battery runs out.

Inverter: The mains power comes to the Inverter. This is
directly sent to the output but the AC is also converted to
DC and this DC is constantly charging the battery. A sensor
and relay mechanism checks whether the mains is ON or OFF.
When the main switches OFF, the relay actuator triggers to
switch from mains to inverter. Rest is same like the UPS.
Because of this sensor and relay, there is a gap between
triggering.

UPS involves more costly circuitary and is therefor more
expensive to make and sell.