If you're not familiar with the Erlang-B model, it is used in the telephony world to predict how many customers will encounter a busy circuit given an average call length and maximum number of concurrent calls.
In the EV world, a "call" is a charging session.
What I found interesting is how low the utilization or efficiency rate of a single station is if you want to avoid unnecessary blocking and how much more efficient the group of stations becomes as you add more stations. This has always been my gut feeling, but I didn't know there was an actual formula for it.
This is just another data-point that really highlights how Tesla gets it. By installing stations at fewer locations but installing more of them (and they typically install 6-8 plugs), they maximize the efficiency of their SuperChargers and minimize potential delays customers might experience plugging in. Also, the fact that two plugs share a single SuperCharger stack also nicely fits in with the ~50% efficiency factor.
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Stalls Capacity Efficiency 2 0.20 10% 4 1.05 26% 6 2.25 38% 8 3.60 45%
I don't know about the rest of you, but one worry I always have when going on a trip that requires QC is that I'm going to find someone else using the spot (or worse finding the station out of order). But if there were more than one QC per location, even if both were in use, your wait time would likely be significantly reduced.
I've only QCd about 10 times. But two of those times I had to wait to use the station.
If you assume a blocking rate of 20%, here's what the capacity/efficiency looks like:
Adding a second plug quadruples your capacity and more than doubles your efficiency. It's a no-brainer. You are much better off installing 2 20-25 kW QC stations over a single 45-50 kW station even with the reduction in charge rate. Why don't Nissan or any of the major charging networks understand this? I think Blink may have been the only other company to realize this as at least they have dual-plug stations (even if they only charge sequentially) and always seemed to install at least 2 if not 3-4 L2 stations at a location.
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Stalls Capacity Efficiency 1 0.20 20% 2 0.95 48% 3 1.90 63% 4 2.90 72% 5 4.00 80%