It's a really simple idea. Imagine a row of houses, oozing heat and light. One of them invites their neighbours for a gathering, reminding them to turn down the power. The power goes down in that street but everyone stays cosy.
A Power Party doesn't have to be in a house. It could be anywhere, in a barn, up a tree, in a school, with the basic idea that when we gather together we share energy and use less at home.