News leaked this week that some code for a pay as you go supercharging infrastructure is going to go live at some point. It looks like Tesla is going to lower the price of at least some of their Model S and X models by making supercharging an optional upgrade.

Some people who have done some math on the cost of supercharging now use $2500 as the cost of the upgrade. Supercharging was an option on the original Model S 60. To upgrade an old Model S 60 now costs $2500, but it cost $2000 originally. Assuming the cost for permanent supercharging will be the same, the calculations should use $2000 instead of $2500. The costs are different in different countries, but the calculations are probably similar for other countries.

Tesla is in a spot of difficulty with some state laws. Some states allow anyone with a charging station to charge by the KWh unless they are a utility. This may be the core reason Tesla is buying Solar City. Solar City is a utility, so after the merger Tesla will be able to charge by the KWh in every US state.

The US average rate for electricity is 12.73 cents/KWh, but they can vary dramatically ranging from a low average of 8.96 cents/KWh in Louisiana to a high of 27.50 cents/KWh. Because of the wide range of costs, Tesla may charge a different rate in different states, but for simplicity of calculation, let’s assume they will charge 16 cents/KWh for electricity at the superchargers. This will allow Tesla to make a little on charging which will cover the expense of maintenance and at least partially pay for expansion.

So if supercharging for the lifetime of the car is $2000 and Tesla charges 16 cents/KWh, where is the break-even point?

$2000 / 0.16 = 12,500 KWh (12.5 MWh)

That’s a lot of energy, but how much driving is this?

If you get about 3 miles/KWh that’s 37,500 miles on superchargers. If you only get 2 miles/KWH that’s 25,000 miles. If you draw on average 50 KWh per supercharger session, that’s 250 supercharger visits.

Tesla could end up charging rates on the order of other services who sometimes have to charge by the minute in states that don’t allow non-utilites to charge by the KWh. Various companies charge different rates, but I’ve found rates ranging from 39 cents/KWh to 79 cents/KWh.

For the sake of calculations, assume Tesla charges 40 cents/KWh, the numbers change a fair bit:

$2000 / 0.40 = 5000 KWh (5 MWh)

At 3 miles/KWh that’s 15,000 miles

At 2 miles/KWh that’s 10,000 miles

At 50 KWh/charge, that’s 100 supercharger visits

Another issue that comes up is if the charge per KWh is too high, supercharging will begin to not be cost competitive with gasoline on a long trip, which is a competitive disadvantage for a long range EV. I made the charts below comparing charging costs for different EVs compared to the cost of gasoline cars. The bottom line is the cost/mile.

At just 30 cents/KWh, the cost per mile for a Model S 90D essentially costs the same for a 25 mpg car at $2.50/gallon for gasoline. For this reason, I believe Tesla will keep the cost/KWh for supercharging low.