Surely it's everyone starting to cook supper at 7pm or so?
Aircon in offices starting as people get into work at 9ish?
Sunset, as everyone turns their lights on?
Think about it for a moment, and try to come up with your own answer before following the cut.
Nope. None of those. The biggest regular demand surge, by far, occurs at the end of EastEnders, when most of the nation puts the kettle on for a cup of tea. Seriously. It's even used for calculating TV ratings.
Other events can cause an even bigger demand. Football matches are worth over 2GW at the end, while the solar eclipse was worth about 3 GW.
To put that in perspective, a good kettle will draw about 3kW of power. That's equivalent to a million kettles, all boiling at the same time.
I've not heard of this happening anywhere else - it seems to be a uniquely British combination of soap operas, electric kettles, and tea.