We often hear that a requirement shall be complete. It means that a requirement statement shall describe the functionality to be delivered. The description shall be sufficient for the designer to understand and implement it.
Why do we need a complete requirement?
Incomplete requirements leave people guessing on what is really requested (how long? 50% of what?). A big risk with incomplete requirements is that the people make assumptions which can lead to improper implementation of the requirement.
On loss of power, the battery backup shall support the normal operation.
The question is for how long the normal operation shall be sustained. When not specified readers will guess what the duration shall be. A possible rephrase can be:
On loss of power, the battery backup shall support the normal operation for one hour.