Electric Vehicles: A Day in the Life

Arizona Electric Vehicle Research Informs Grid Planning and Operations

By Mary Beckman

With more electric vehicles (EVs) on the road every year, power companies need to plan for the power capacity needed to charge all the batteries.

“Our gas stations are moving to our houses,” said EPRI Data Scientist Jamie Dunckley. “Utilities need to provide energy whenever drivers want it.”

But power companies have limited data on how their customers use EVs. How many miles do they drive, how many kilowatt-hours do they consume, and when do they charge? Would they be willing to charge at off-peak times? Utilities need such information for planning.

EPRI’s Dunckley worked with Salt River Project (SRP) and their EV-driving customers to answer these and other questions. Researchers developed a comprehensive picture of EV use by deploying data loggers on 100 vehicles owned by SRP customers. Over 18 months, the data loggers tracked where, when, and how much customers charged, how much energy they consumed while driving, where and how far they drove, vehicle speed, and other data.

Each EV consumed 2,700–3,300 kilowatt-hours per year at a cost of $324–$396 (based on a national average electricity price of 12 cents per kilowatt-hour). For non-Tesla owners, more than 80% of their charging sessions were at home compared with 63% for Tesla owners—who could charge for free in 30 minutes at public “Supercharger” stations.

SRP offers three rate options: a flat rate plan, a time-of-use plan with higher rates in the afternoon and evening, and a time-of-use plan with much cheaper rates between 11 p.m. and 5 a.m. More than half selected time-of-use plans and consumed six times more electricity for overnight charging relative to daytime charging, indicating that cheaper off-peak rates can shift load.

“Even Tesla drivers with $100,000 vehicles were incentivized by saving a few dollars,” said Dunckley.

Most charging occurred at Level 2 (74%), followed by Level 1 (23%) and DC fast charging (3%). Charging level proportions varied widely by EV model. For example, DC fast chargers provided LEAF owners with approximately 2% of their energy while they provided Tesla owners with about 11%.

“I was surprised to see how the different EV models used electricity differently,” said SRP Principal Environmental Scientist Kathy Knoop.

Teslas used energy less efficiently than other models. “We chalked that up to Tesla drivers showcasing the vehicle’s performance with the ‘Ludicrous’ acceleration mode,” said Knoop.

Knoop expects the logger data to be valuable for utility load forecasting. “The more information we have on when people charge their cars and how much energy they’re using, the more we can refine the energy forecast, which reduces cost for our customers,” she said.

“The study opened our eyes to new uses for the data loggers,” said Knoop. “We found an innovative approach to demand response.”

Drawing on information from the data loggers, utilities can offer a rewards program, similar to those offered by grocery stores, gas stations, and credit cards. For example, if there is excess solar generation on a sunny afternoon, a rewards program could use a logger’s GPS data to determine a car’s location and send a text or e-mail to the driver offering a gift card for charging at a nearby station.

“It gives customers different options outside of a pricing plan,” said Knoop.

Some utilities have started to make such offers. ConEdison’s SmartCharge New York program rewards customers for charging at certain times of the day.

While it’s true that our gas stations are moving to our houses, EV drivers may increasingly have ways to get out and follow the sun.

Artwork by Ariel Davis

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