The EPA limits gasoline filling speeds to 10 gpm (gallons per minute), or 0.63 l/s (liters per second). The volumetric energy density of gasoline is about 36 MJ/l (megajoules per liter), which means that, at the gas station, energy is flowing into your tank at a rate of 36 MJ/l * 0.63 l/s = 22.7 MJ/s = 22.7 MW (megawatts).

Now, suppose that the battery-to-wheels efficiency of an electric car is five times the tank-to-wheels efficiency of a gasoline car, which is a fairly reasonable assumption. Then we only need a

*charging power*of 22.7/5 = 4.5 MW. Generously supposing 90% efficiency from grid to battery, we need “only” draw

*five million watts*off the power grid.

That is an absolutely vast amount of electric power: not much less than is needed to supply 4,000 American homes. This is not a plug-in device.

A reasonable rule of thumb for distribution level electricity supply is “100VA/kV” in other words, if you were to be a 10kV primary customer, you can expect to be able to draw at most 1 MVA (the difference between VA and watts is not important for the current discussion). On that basis, the 5MW charging station will need to be a 69 kV subtransmission customer of the local power utility.

*One*charging station, not a gas station forecourt with 8 of them.

Now, let's talk about slew rate. You can't just turn on a 5 MW load (for want of a round number) like a 60W lightbulb. Call your local power company and ask them how quickly they would allow a subtransmission customer to turn on a 5 MW load. The answer cannot be faster than they can spin up a gas turbine. The slew rate of a “hot” GE gas turbine generator is, at most, 5%-per-minute. In other words, to be able to even ramp-up a 5MW charger to full power in one minute would take 100MW of spinning reserve, and only

*after*that have you hit gas pump-equivalent power.

Let's look at that another way. Take the 85kWh (kilowatt hour) battery in the top-end

*Tesla Model S*. 85kWh is 306MJ. Suppose you want to charge that sucker in a minute flat, which is not unreasonable given its range of 265 miles: one minute would give you ten gallons of gas, at least enough to run a modern luxury car for that distance. To supply 306 MJ in 60s is 306/60 = 5.1 MW. Pretty much the same number.

In synopsis, refuelling an electric car at the same rate as a gas pump, any way you look at it, requires something of the order of 5 MW of electric power. This is, practically speaking, impossible. Notice that I haven't mentioned

*cost*. Economically speaking, it is utterly beyond any reason: the charger alone would cost millions. Recharging an electric car at even ten percent of gas-pump equivalent speeds (requiring “only” a half-megawatt charger), presents enormous technical challenges in electricity supply.

In short, the technical challenges of recharging an electric car on consumer-acceptable timescales is almost nothing to do with the car or its battery.

I feel like this is a discussion that is desperately needed with all the bulls currently behind Tesla. Maybe you could do a write up on the metal air battery, many are thinking this will be available in a Tesla soon (3-6yrs). From what I've read, that's being enormously generous if we're talking implementing it in a safe way.

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