Summary: Demand Charges can dominate the cost of a business offering EV charging, and the true costs can be shocking...
Most of us are used to looking at our home electricity costs as a pure energy cost that is measured in dollars per kiloWatt-hour ($/kWh). These can vary a lot, with the national average about $0.1264 per kWh, but ranging from $0.0882 to $0.3054 per kWh. Source:
EIA - Electricity Data
Business electric costs in most places are based on energy cost ($/kWh) plus demand charges ($/kW). See
What is the demand charge on my business account electricity bill? for a good explanation of demand charges. The justification for demand charges is that a large portion of the cost of producing electricity is the cost of capacity needed for peak demand, and demand charges bill for that peak demand of each account. In the U.S., this is usually measured as the average power used in the peak 15 minute period of a billing cycle and is measured in kW. Demand charges are then billed as Dollars per kiloWatt ($/kW).
I am helping with putting in EV charging at a Community Center in Boulder, CO, and want to understand the true cost of offering charging to users of the Community Center. Xcel Energy is the electric utility in Boulder and the commercial rate tables are at
http://xcelenergy.com/staticfiles/xe/Regulatory/COBusRates.pdf. Use rate SG for a large commercial account. Because there are many surcharges, taxes, fees, etc, on an electric bill, I asked a friend to get a copy of an actual commercial bill at one of the retail buildings he manages. After putting that data into a spreadsheet and matching the bill to better than a percent, I derived the actual, total, energy cost as $0.046942 per kWh and an actual, total, demand charge as $23.66 per kW.
To get a handle on the cost per charge, cost per kWh for charging, and cost per month, I put a spreadsheet together using those energy and demand charges, based on offering 50 charges a month with each charge using 30 kWh
(about 90 rated miles for a Tesla). I then compared two L2 EVSE offerings (208 Volts with 40 and 80 Amps available), a full power CHAdeMO, and for grins, a Supercharger stall. I assumed worst case that the EV charging would directly increment the max kW, Demand of the site. For hotels, this may be on the high side, because most charging is done in the evening, when other demands like air conditioning are lower, but I had to start somewhere. For places like a Community Center, there is a good chance that peak charging will happen at other peak usage times during the day.
Below are the results. You can see the original spreadsheet and equations at
Sample Xcel Electric Bill - Google Sheets
View attachment 86364
Most of the rows should be pretty obvious except the last row. Because I am looking at this charging being at a non-profit community center, "Endowment" is a measure of what size endowment would be needed, producing 5% a year, to support the cost of offering free charging.
Notice that even for the 40 Amp J1772 case, Demand Charges dominate. At higher charger powers, the Demand Charges are huge. Running a CHAdeMO can cost over $1,000 a month and a Supercharger Stall is almost $3,000 a month in Demand Charges!!!
Even with the proviso that these Demand Charges are worst case, I find these results shocking, and at this point, I do not plan on recommending that anything more than 48 Amp J1772 charging equipment be installed. In fact, I may recommend 30 Amp J1772 for this application. It is an in-town location, unlikely to help any long distance Tesla's. Supporting the Leaf class of EV is good, and why open the community center to a lot of excess costs.
After doing these calculations, I certainly see why Tesla is working on Energy Storage to clip Demand Charges at Superharger Sites, and push towards the average kW usage. BTW, I have already filled out the business usage form at the Tesla web site for Tesla Energy for Business. No reply yet, but I look forward to seeing what they may offer.
I would like to be wrong in my calculations, but have not found an error yet. Please help me find a way to save on the electric bill and still offer high power charging!
