[jaywalk]

> If the grid is very slightly over 50hz it means demand exceeds supply and vice versa.

Not to be too pedantic, but it's actually the opposite. Frequency drops as demand goes up, which requires more supply to compensate. This is because the generators physically spin more slowly as they are put under more load.

[adwww]

thanks, I knew I should have googled to check...

Also occurs to me that it's 60hz in North America, but point roughly remains!