Volt Amps and Watts are both a determination of instantaneous power, a 650 W UPS does not necessarily have 650 Watt hours of power stored in it, it may only have 200 Watt hours of power stored in it but can deliver up to 650 Watts at peak draw, it wont last very long but it can support a bigger item temporarily.
What UPS do you have? Look at the up time it promises for given loads, that will help you determine what it would be for such a small load.