Inverter and UPS are NOT the same thing!
UPS means Uninterruptible Power Supply. A UPS is used in situations where the switchover time is critical. A UPS can detect power loss and switchover to batteries in less than 25 milliseconds. An inverter cannot switchover in such a short time.
Thus a computer which is running while connected to an inverter will restart upon power loss because the inverter does not have the capability to switchover to batteries fast enough. But the same computer will keep running if it is on the UPS. Thus a UPS is always more expensive than an inverter.
UPS is essential for medical equipment, computers, surveillance systems etc. For fans, lights etc an inverter can do the job.