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Per-minute supercharging cheaper than home electricity? (NJ)

I'm based in NJ and recently did the following calculation based on the given values:

- I pay $0.18/kWh at home (supply + delivery)
- I pay per-minute at superchargers in NJ ($0.26 or 0.13)
- At superchargers, my car will charge, at minimum, (from 0-60%) at 350miles/hr. I usually get ~500miles/hr.
- Last night, I charged my car for 15 hours and got ~70 miles. I paid $3.78 (1.4kWh*15hrs*$0.18/kWh)
- If I charged my car at the supercharger, I would have only paid (350miles/hr / 60 minutes = 5.83 miles/minute, 70 miles/5.83 miles/minute = 12 minutes. 12 minutes * $0.26 = $3.12) given that my car was charging in the 'sweet spot' of 0-60%

Am I missing something, or is it cheaper for me to charge at superchargers (versus home) in NJ, given these params?
 

mociaf9

Active Member
Oct 18, 2018
3,207
6,701
CA
I'm based in NJ and recently did the following calculation based on the given values:

- I pay $0.18/kWh at home (supply + delivery)
- I pay per-minute at superchargers in NJ ($0.26 or 0.13)
- At superchargers, my car will charge, at minimum, (from 0-60%) at 350miles/hr. I usually get ~500miles/hr.
- Last night, I charged my car for 15 hours and got ~70 miles. I paid $3.78 (1.4kWh*15hrs*$0.18/kWh)
- If I charged my car at the supercharger, I would have only paid (350miles/hr / 60 minutes = 5.83 miles/minute, 70 miles/5.83 miles/minute = 12 minutes. 12 minutes * $0.26 = $3.12) given that my car was charging in the 'sweet spot' of 0-60%

Am I missing something, or is it cheaper for me to charge at superchargers (versus home) in NJ, given these params?
You are making the math much more complicated than it has to be. Use kW and kWh to compare, not miles:

1 kWh/$0.18 * $0.26/min * 60 min/h = 86.66... kW. & 1 kWh/$0.18 * $0.13/min * 60 min/h = 43.33... kW.

So the periods where your charging on the supercharger will be cheaper than home charging will be [44 kW, 60 kW)U[87 kW, max] (i.e. if you're supercharging at 44 kW up to 60 kW, not including at 60, OR from 87 kW to the max charge rate). Outside of those ranges, it is cheaper (or, at two specific points, equal price) to charge at home.

Note, this direct price comparison is slightly skewed by the fact that not all the energy you're getting billed for is actually making it to the battery as energy that is usable for driving. And this is exacerbated by the fact that it appears you're home charging from a regular 120V/15A wall outlet. The lower the power you charge at, the higher the percentage of non-motive charging losses will be and you're using basically the lowest power possible. This has the effect of making the energy from the higher powered supercharger more "valuable". So, the true ranges will be a bit wider than actually calculated.

Of course, the big thing your direct price comparison is missing is that it doesn't account for the value of your time (or any extra driving that you need to do to get to/from the supercharger). If you calculate the actual dollar savings by supercharging vs home charging and look at the time investment, you'll likely find that you'd be "paying yourself" slave wages by doing it. But, you do you.
 

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