Chris Mooney:

Clearly, a lot of EVs means a lot of electricity use. In particular, if large numbers of EVs are charging at roughly the same hour of day — and that time of day is likely to be when people get home from work, which is when electricity use spikes already — the repercussions could be massive. Electricity is most expensive during these peak hours, and power companies have to fire up “peaker plants,” usually driven by natural gas, to slake demand.
 “If it increased the peak we would have to invest more money for more generation capacity for the peak, which is the most expensive kind of generation capacity,” says the Rocky Mountain Institute’s Chris Nelder, one of the authors of the report.