Bidirectional home chargers


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First Name
A. J.
Aug 1, 2019
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First Name
A. J.
Tesla X Extended Range Plus 2019, Lexus, Landcruiser, SR5
EE Retired
Demand charges are for the peak kW draw during any given 15 min period and are assessed per meter and per billing cycle.

Using the data linked:
General Lighting and Power (Schedule 2),
If during the month, the maximum draw at a Supercharger meter was 500 kW (4 simultaneous vehicles charging @ 125 kW), the monthly demand charge would be 500* $2.40 or $1,200. Add in $25.35 meter charge and 2.35¢ per kWh.
Using that info let's suppose that on average there is only 1 car charging at 125 kW hr but in one 15 min interval there were 4 so that the peak 15 min demand was 500 kW. Thus the demand charge would indeed be $1200, But this, you say, is assessed per billing cycle. If the average demand is 125 kW in a 31 day billing cycle the station will use 31*24*125 = 93,000 kWh. So the total charge will be $23.35 + $1,200 + $0.0235*93000 = $3408.85. Now the demand charge of $1200 is more than 1/3 of the bill but the rate is 3408.85/93000 = $ 0.0366543 /kWh . That's only a little more (a third) than the basic rate $0.0235. So I am still confused about the $2.86

Demand charges are more of an issue for infrequently used charging stations. A station that used just a few times a month could have a demand charge split between only a few charging sessions. A busy Supercharger will have it's demand charges split among hundreds of sessions.
Roger that. They are imposed to encourage users to level their loads. The example numbers I gave were for a load factor of 4. To get a per kWh of 2.86 would require that the load factor be 861 meaning a total monthly usage of 431 kWh meaning, for example, that 17 cars used that station in a month to take on an average charge of 25 kWh but that 4 arrived in the same 15 minute window. Two hundred sessions at 25 kWh each implies a load factor of 74.4 for

Average Load 6.7 kWh; Cycle use: 5000.0 kWh: Per kwh $0.2686; Demand charge Per kwh $0.2400
Total bill $1342.85; Demand bill: $1200.00

Were I Tesla with historical data on my stations I would take action to keep my load factor down by limiting available juice to individual charging cars when the station is unusually full. This is called "load leveling" and is exactly the behaviour that demand charges are intended to encourage.