ok, i got it. you need not keep the main switch on always. if you use the ups in case of powercuts then you need to keep the main switch on till the batteries are fully charged. example - if there was no powercut during your use of the pc, then you can switch off the mains too. If there was a powercut for 10mins, then the battery would have lost around 20percent of their capacity and to charge them, you can switch off the ups front button, but need to keep the main switch to the ups always on till it charges fully. This can take anywhere between 1hr -3hrs depending on the charge lost.
Since your ups model does not support pc monitoring, you should as a safety, shutdown the pc after 15mins of prolonged power failure. This would be somewhere near the 40 percent battery usage i assume.