When the Tesla factory in Austin, TX is completed next year (2021) the production of the Semis will begin. Initially Tesla will be lucky if they make 100,000 trucks the first year. It might be awhile before they can produce 200,000 to 300,000 trucks per year until their battery production can increase and keep up. With a range of 621 miles (with the new 4860 cell) they will not be stopping very often to charge up.
It will be at least five years before there are 1,000,000 Tesla Semis on the highways. I do not expect one million trucks to have a negligible impact on the power grid anytime soon. Tesla will be rolling out battery power cells at all charging stations and that will reduce the need to turn on Peaker power stations. The charging station batteries can charge during low usage hours between midnight and 8 am. If Tesla can ramp up production to 500,000 to 1,000,000 trucks per year in the next five to ten years there will plenty of time to increase energy production.
The semis will most likely have four 100 Kwatt battery packs and have several ports to speed up charging. In thirty to fifty minutes the driver is back on the road again. That is enough time to get a bite to eat and use the facilities. Right Scotty?