Offline Uninterruptible Power Supplies are also often called Standby types. They are so called as when the mains supply is normal, they do nothing, i.e. they are offline (or standing by!).

The output voltage is exactly the same as, or dependent upon the input voltage. For this reason, such systems are now called VFD for Voltage, Frequency Dependence.

The mains constituents of the offline UPS can be seen in the diagram below:

When the mains supply goes to an unsafe level (either too high or too low), they will disconnect from the mains and start supplying the load from the inverter. The inverter in standby systems is nearly always a square wave, due to cost reasons.

During the transition from mains operation to inverter there is a time delay of around 10-20ms. It's not long and most PC's won't notice it (much) but other equipment may have issues with it.

Don't expect much from an offline UPS. They are more suited to protecting against annoyance power cuts for a low grade PC than for anything remotely approaching important.