Prices are fixed within each state in the US and within each country overseas. You'll pay per kWh in most locations, though some areas require customers to be charged per minute instead. The company's new FAQ section lists prices per state: California residents, for instance, will have to pay $0.20 per kWh, while those in New York will pay $0.19 per kWh. The section also explains that unused credits don't roll over to the next year, so you might as well use them all up.

The company said in a statement:

"To put the affordability of Supercharging into perspective, customers will pay about $15 for a road trip from San Francisco to Los Angeles, about $120 from Los Angeles to New York, about €60 from Paris to Rome, and about ¥400 from Beijing to Shanghai."

According to Tesla's research, 400 kWh is enough to power one of its cars for 1,000 miles and can cover the driving needs of most owners for a whole year. If you don't do a lot of long-distance driving, you might never even have to pay for extra charging.