Precisely, even with that significant slack, it still happens. How frequent will be the overloads in a system with no energy storage capacity, which is sized to

Precisely, even with that significant slack, it still happens. How frequent will be the overloads in a system with no energy storage capacity, which is sized to handle the existing load peaks and already has infrequent overloads? The average commute in the US is 50 Km per day and if that is the average it is reasonable to assume some communities will be highly correlated above average, for example if they are 40-50 Km away from a large economic hub, a large proportion of residents will drive the 100Km roundtrip per working day. The lower price of electric comute will prompt many residences to have two electric cars driving the average every working day, for a total, again, of 100Km = 20KWh. So double digit is not exceptional, it will be the average, an extra 3KW of power for each household for the entire night, assuming an outstandingly smart grid that can perfectly level off demand. Combine that with rare events like hot nights and large movements, psychological reactions like panic, and you have an unreliable local supply that might trip every few months without expensive upgrades.