A power cut was a loss of connection to electricity.
Sometime in the 1970s, there were several consecutive power cuts. People "took advantage of the dark", and many people had sex on these days. As a result, nine months later, there was a "baby boom".
Another such power outage occurred during Miracle Day while Gwen Cooper was trying to research the Miracle. She and her husband Rhys too "took advantage of the dark". (TV: The Gathering)