Just came across this on Sky News. The US doesn't seem to be the only nation that has a power problem. But, of course, we don't know yet what exactly happened.
A Tube spokesman said: "There has been a National Grid failure which has affected the underground.
The failure is thought to have happened at around 6.15pm.
A spokesman for electricity network operator EDF Energy said: "We have lost supplies to large parts of south London in the last few minutes as a result of a National Grid failure supply in the south London area.
UPDATE: Seems like the power is back on in London.
34 minutes ago