The clock runs at exactly 50Hz (in the UK) which is the frequency that the mains provides power at to each house. Power stations are closely monitored to remain at 50Hz so that when extra power is needed in the grid the power is provided at the same frequency timing from other power stations coming on line - this accuracy is a real bonus for synchronous clocks.
No batteries to change!

Over a long period of time this makes these clocks very accurate.

The power consumption is about 2 Watts. That is approx one fifth of an energy efficient lightbulb

Check out the current Real Time Mains Frequency here...