The team that traveled through would only be able to return if they located a ZPM (zero-point-module).
They couldn't maintain the City's power with all the ZPM's (Zero Point Module) drained.
The Daedelus provided the Atlantis Expedition with a link to Earth and a ZPM to power their shields.
The word usage examples above have been gathered from various sources to reflect current and historial usage. They do not represent the opinions of YourDictionary.com.