DOE releases report from 6 projects evaluating EV charging impacts on grid and customer charging behaviors

21 December 2014

Charging patterns with (TOU) and without (RES) whole house time-of-use rate during summer weekdays at Progress Energy, one of the participating utilities. (Peak period is in gray.) Click to enlarge.

The US Department of Energy (DOE) has released a report detailing the findings from six utilities which evaluated operations and customer charging behaviors for in-home and public electric vehicle charging stations. The work was done under the DOE’s Office of Electricity Delivery Energy Reliability’s (OE) Smart Grid Investment Grant (SGIG program).

This report provides the results of these SGIG projects to help individual utilities determine how long existing electric distribution infrastructure will remain sufficient to accommodate demand growth from electric vehicles, and when and what type of capacity upgrades or additions may be needed. The report also examines when consumers want to recharge vehicles, and to what extent pricing and incentives can encourage consumers to charge during off-peak periods.

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The electric power industry expects 400% growth in annual sales of plug-in electric vehicles by 2023, which may substantially increase electricity usage and peak demand in high adoption areas. Understanding customer charging patterns can help utilities anticipate future infrastructure changes that will be needed to handle large vehicle charging loads.

The six SGIG projects evaluated more than 270 public charging stations in parking lots and garages and more than 700 residential charging units in customers’ homes. Participating utilities were:

• Burbank Water and Power (BWP)
• Duke Energy (Duke)
• Indianapolis Power Light Company (IPL)
• Madison Gas and Electric (MGE)
• Progress Energy (now part of Duke Energy as a result of a merger in 2012)
• Sacramento Municipal Utility District (SMUD)

Because there are relatively few plug-in electric vehicles on the road today, the SGIG projects focused on establishing the charging infrastructure with a relatively low number of stations and evaluated a small number of participating vehicles.

Although project results showed negligible grid impacts from small-scale electric vehicle charging today, they also gave utilities important insights into the demand growth and peak-period charging habits they can anticipate if electric vehicle adoption rises as expected over the next decade.

Major findings were grouped in three categories: charging behavior; grid impacts; and technology issues.

Charging behaviors. The studies found that the vast majority of in-home charging participants charged their vehicles overnight during off-peak periods. Where offered, time-based rates were successful in encouraging greater off-peak charging.

Public charging station usage was low, but primarily took place during business hours and thus increased the overlap with typical peak periods. Plug-in hybrid owners frequently used the (often free) public stations for short charging sessions to “top off their tanks.”

Grid impacts. Length of charging sessions and the power required varied based on the vehicle model, charger type, and state of battery discharge. While the average power demand to charge most vehicles was 3-6 kW (roughly equivalent to powering a small, residential air conditioning unit), the load from one electric vehicle model can be as much as 19 kW—more than the load for most large, single-family homes.

Technology issues. Installing a 240-volt charging station, which typically charges 3-5 times as fast as a charger using a standard 120-volt outlet, requires a licensed electrician and occasionally service upgrades. Public charging station installation had high costs and required substantial coordination with equipment vendors, installers, and host organizations to address construction, safety, and code requirements.

In addition, low usage at public charging stations will require longer capital cost recovery without substantial growth in usage.

Some utilities also found residential interoperability problems in communication between smart meters and charging stations. SMUD found that the two devices only connected successfully about 50% of the time during load reduction events.