Sorry chaps - we are forgetting something.
Provisioning has always been based on a combination of statistical analysis and common sense. There will never be the capacity to cater for a worst case scenario. The electricity network is based on provisioning rules that worked very well until now. The planning that Tim references will change the provisioning rules but will still make reasonable assumptions. For example, most car charging will take place when people are asleep and most cooking occurs when people are awake. Some limited intelligence in domestic installations will cater for usage anomalies. The planning already foresees cars being used as a reservoir that will feed back to the grid at peek usage times (evening meals etc) and defer charging until later on.
