This is from the UK perspective.

You might like this webpage for details and explanation: Renew-reuse-recycle

Quote:
Note: The normal system frequency is 50Hz. As electricity cannot be stored, the instantaneous generation must match the demand being taken from the system. If the instantaneous demand is higher than the generation, the system frequency will fall. Conversely, if the instantaneous generation is higher than the demand, the frequency will rise. System frequency will therefore vary around the 50 Hz target and National Grid has statutory obligations to maintain the frequency within +/- 0.5Hz around this level. However, National Grid normally operates within more stringent 'operational limits' which are set at +/- 0.2Hz.

Europe is now standardised on 230Vac at 50Hz

The standard for 50Hz demands that the AVERAGE frequency is 50Hz exactly over a period of time, I think 12 hours so frequency controlled clocks are self correcting but at any instant may be a little fast or slow, un-noticable.