I was only asking because the tiered supercharger rate seemed arbitrary. (How or why did they pick 60 as the half price cutoff)
Wisconsin rate (my home state)
$0.20 per minute above 60 kW
$0.10 per minute at or below 60 kW
But if the short range version supercharging maxes out around 60kW then it makes some sense, as people might feel confused or cheated somehow if they charged at say 65kW and pay double the price as opposed to sub 60kW. We have to remember tesla is moving into the mass market here some people will drive across town to "save" $0.01 on a gallon of gas. It will be interesting to see when the sr model 3 hits the road if it is infact caped at or below 60kW as a maximum rate of charge.
Key thing is those are not per kwh rates, those are per min rates. 60kw may be arbitrary, but at least at 60kw the math is easy, each minute charges one kwh, so $0.10/min = $0.10/kwh. But you'll almost never pay exactly that.
Let's say the "worst-case" is the SR has a max rate of 62kw supercharging, putting it just into the higher tier - well, it's hard to stay at that 62kw max rate for more than 15-20 minutes, because the SoC builds up, and the taper starts. Once you hit the taper, the taper is fairly continuous - so it's not like you're at 60kW for the rest of the charging session - you'll be doing 59kW for a minute or so, then 58, 57, 56, but still paying $0.10 per minute. So you only truly pay the best rate of $0.10/kwh for one minute.
If you're road-tripping or long-commuting, generally you don't want to hang around too long on the taper, so the $0.20/min is all you'll consider. If you're in the new urban dweller scenario where the SC is your primary fueling station, and you really need to "top off" to 90% of more for the week ahead, then after 30 minutes you might be tapered down to 40kw and are actually paying $0.15/kwh, by 45 min you may be at 20kw and are paying $0.30/kwh at that point, and waiting longer for each kwh to boot.
And due to the way the Superchargers work, even pulling in with an empty battery, many times you'll never see above 60kW for one minute let alone 15-20 minutes. More likely instead of people happy they're only paying "half-price" for those electrons, they'll be complaining here about the slow Supercharging speeds they're getting.
