According to a recent PSR Analytics report that measured Summer 2013 vehicle charging in the nation’s highest residential concentration of electric cars, electric vehicle (EV) owners are charging their cars much less during hot summer afternoons than most behavioral models predict. The analysis, conduced by Pecan Street Research Institute, found that not only is charging behavior is much more diverse than has been predicted, but represents a much more manageable energy load and may be highly elastic to time-of-use pricing and similar tools.

If such findings are confirmed by other research, it could significantly increase utility industry estimates on the number of EVs the electric grid could handle without triggering disruptions or requiring major system upgrades.

Pecan Street’s EV research trial in Austin, Texas has what appears to be the highest U.S. concentration of electric vehicles, including over 50 in a single half-square mile neighborhood. The institute’s data-intensive research trials (currently in three states) has produced the world’s largest research database of residential energy use according to the company.

“EVs represent the largest new electric load to appear in homes in a generation,” said the report’s lead author, Pecan Street CEO Brewster McCracken. “We still have a lot of consumer research ahead of us, but these findings suggest that this new load is not only manageable, but movable.”

Summer afternoon charging has emerged as a point of focus for utilities, particularly for those serving large cities where clusters of EVs have started to appear in some neighborhoods. The concern among utilities is that homeowners with EVs will all charge upon arriving home in the afternoon, and that such charging would occur during the peak demand hours on summer afternoons. During those periods, electric grids in many parts of the world are stressed due primarily to air conditioning loads coming on as people arrive home.

The PSR Analytics report analyzed over 2,500 vehicle charge events between June 1 – August 31, 2013 in a randomly selected subset of 30 homes participating in the institute’s EV research trial. The measured vehicles consisted of 21 Chevy Volts, nine Nissan Leafs and one Tesla Model S.  All participants are part of Pecan Street’s broader energy research. Fifteen of the participants are also part of a research trial examining the impact of time-of-use energy pricing. The other 15 analyzed homes are not part of any EV behavioral intervention.

Key findings from the report include:

• For the drivers who are not part of the pricing trial, only 22 percent of weekday EV charging occurred during peak electric demand hours (3-7 pm).  The rate was even lower among the EV drivers participating in the pricing trial — just 12 percent of their charging occurred during peak demand hours.
• The average weekday “charge duration” for participants was just under two hours per charge. That is consistent with approximately 20 miles of driving between charges.
• Using national average electric rates, it costs EV drivers less to charge their cars for a month than it costs to fill up a Prius once.
• Even with these surprising findings, EVs’ impacts could prove more challenging for utilities in regions without significant air conditioning use. In those regions, utility distribution systems are frequently sized for homes with flatter and smaller electric use patterns. These systems could be stressed over a broader range of hours each day by new consumer products like EVs with high instantaneous electric draws.
• Vehicle telematics (OnStar for Volts and CarWings for Leafs) played a significant role in customers’ shifting of charging to late night/early morning hours among the pricing trial participants (and a handful of the non-participants).