An important measurement of your vehicle’s efficiency is understanding the cost per mile of your daily driving. For a gasoline vehicle, one merely divides the cost of a gallon of gasoline by the miles-per-gallon the vehicle gets to determine cost per mile. As we move into the electric vehicle era, determining a vehicle’s operating cost becomes more complicated. That’s because an electric vehicle’s cost per mile can depend on many factors that influence what you pay for charging its batteries – the price of electricity, the length of time it takes to charge, time of day, how close to ‘full’ the battery is, and even an EV’s onboard charger capabilities. Cost can also vary considerably based on whether you charge at home or at public chargers.
We’ll guide you through the process of understanding electric vehicle charging and how this directly impacts driving costs. Just a note, though, that our calculations focus on battery electric vehicles (EVs) and plugin hybrid electric vehicles (PHEVs) when running solely on battery power. Because things get more complicated when the gasoline engine of a PHEV is operating, this is not covered here.
CRUNCHING THE NUMBERS Electric vehicle energy use is measured in terms of kilowatt hours per 100 miles (kWh/100 miles). This would be like gallons per 100 miles in a gasoline vehicle. The Environmental Protection Agency (EPA) includes this number on the window stickers of plug-in vehicles along with their estimated miles per gallon equivalent (MPGe), since we’re so used to a gas vehicle’s mpg rating as an efficiency reference. EPA determines MPGe by assuming a gallon of gasoline is equivalent to 33.7 kWh of electrical energy (MPGe = 3370/kWh/100).
So how do you determine what each mile of driving costs in your electric vehicle? Let’s do an example. The cost of electricity in a sample California city is about 15 cents per kWh ($0.15/kWh). If a current model Kia Soul Electric with an EPA rating of 31 kWh/100 miles was charged here, it would cost $4.65 to travel 100 miles. This translates to $0.15/ kWh x 31 kWh/100 miles = $4.65/100, or 4.65 cents per mile.
Gasoline prices in the U.S. vary considerably depending on markets and world events. In recent times, that range was between $3 to $4 per gallon, while the average price of electricity ranged from $0.095/kWh in Louisiana to $0.31/ kWh in Hawaii. Even within a state the rate depends on what a specific utility charges, which can differ substantially. Thus, the cost to drive an electric Kia Soul could range from 2.95 to 9.6 cents per mile. In comparison, the cost of driving a gasoline Soul could range from 10.0 to 13.3 cents per mile.
CHARGING AT HOME Unlike gasoline, the price of electricity can vary not only by location, but the time of day it is used. Utilities typically have two types of rate plans – level-of-use and time-of-use. With level-of-use, the price rises with the amount of electricity used. Here, the last kilowatt used in a month could cost more than the first one, which would most likely be the case for electric vehicle owners. With time-of-use, utilities divide a day into peak, off-peak, and sometimes a mid-peak period. Some utilities have as many as six time-of-use periods. In any case, electricity is most expensive during peak usage times, usually in the morning, late afternoon, and early evening. Others offer a lower rate for EV charging than the rest of a home’s electrical service, but the savings may not amortize out considering the fee charged for installing a separate meter. Additionally, many offer the option of a special EV rate plan that can make the cost of charging an electric vehicle more financially favorable.
You can charge an EV or PHEV using Level 1 household 110 volt current using a portable charger often provided with a plug-in model, with the charger powered via a standard wall outlet. Typically, electricity is supplied at a 1.4 kW rate. This is workable for topping off batteries after limited daytime driving where little battery power was used, but the time required for charging a fully depleted battery can be considerable. For example, to charge a Chevy Bolt’s 66 kWh battery to 80 percent state of charge (SOC) with Level 1 charging would take about 38 hours…far too long for most drivers. This time is reduced to about 7 hours with a Level 2 charger at 240 volts and a 7.2 kW charging rate. Level 2 charging is recommended for any vehicle with a battery capacity larger than 10 kWh.
While the latest generation EVs and some PHEVs have the capability to fast-charge to 80 percent SOC in a half-hour or less at a Level 3 and above charging rate, Level 3 charging is not available for homes since this requires 480 volt electrical service. In all cases it’s important to avoid discharging EV batteries to near-zero percent SOC to avoid diminishing battery longevity.
Charging at home at a more convenient Level 2 rate requires special Electric Vehicle Supply Equipment (EVSE). These wall or portable chargers cost between $200 to $1000, with wall chargers also requiring installation that can run from $800 to $1300. Most automakers offering EVs and PHEVs have a recommended EVSE provider, but there are many companies selling EVSEs.
In penciling out the financial benefit of a plug-in vehicle, your number crunching should include the cost of the EVSE. For example, if an EVSE costs $1500 installed and you plan to drive an EV 75,000 miles over a five year period, the EVSE’s amortized cost will be 2 cents per mile. Since most people will likely drive their EV for many more years, amortized EVSE cost could be much lower.
While the overall cost of driving electric can vary widely depending on vehicle purchase or lease cost, electricity rates, EVSE and installation cost, and the length of time an EV is driven, as a general rule owning and operating an EV will be less than that of an equivalent gasoline vehicle.