The average passenger car EV uses ~350watt-hours/mile. (Note that this is actually a high estimate, the Model 3 uses 250-300wh/m) Assuming 50 miles a day, charged every night, that's 17kwh/night (or $2.04 @ 12c/kwh).
Looking at California right now, they have a 8GW spread between night time low and daytime high. With another 10GW of available capacity over the daytime high.
(kind of neat charts here:
http://www.caiso.com/outlook/SystemStatus.html)
Someone please check my math, but 8GW of capacity divided by 17kw is 470 thousand cars.
This assumes that each of those 470k cars plugs in at 3am and charges at 17kw for one hour. So the load on the grid spikes to the daytime high for one hour, then goes back down to the normal night time load.
Of course, that isn't realistic.
Let's assume that 17kw is drawn over 5 hours, for an average of 3.4kw/hour.
Now the grid can support 2.3 million EVs overnight (8GW/3.4kw). Still leaving the extra 10GW of reserve capacity over the daytime peak.
The Tesla semi is expected to have no bigger than a 1MWh battery (but probably a bit lower). Assuming a 5 hour charge time, that's 200kw over 5 hours. Or, 40,000 trucks that each drove the full 500 miles the previous day.
Or, they can charge during the day and soak up cheap solar power.
Thanks for making me crunch the numbers. I have no doubt that the current grid,
tonight, can handle as many cars as we can throw at it, especially if we incentivize it with Time Of Use charging.