My dad built one of these: https://www.youtube.com/watch?v=iad37q9kNPk and we spent a lot of time looking at our house power and trying to figure out where recurring patterns came from. Back in the early 70s, refrigerators and air conditioners (things with high-load motors running intermittently) were often the culprit.

He built a bunch of Heathkit products. We had an intercom system that worked over the power wiring. It quit working before I was clever enough to use it to play jokes on my little sister.