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It’s well understood that driving electric is more efficient with a lower cost-per-mile than driving internal combustion vehicles. That’s especially true if you charge an EV up at home. But what if you need to use public chargers on the road or live in an apartment where a commercial pay-per-use charger is your only option?

The cost can vary significantly since commercial chargers use different methods of payment. For example, many providers charge for the time it takes to charge a battery rather than the kWh of electricity delivered. This would be like gasoline stations charging for the length of time a nozzle dispenses gas in the fuel tank, not by the number of gallons of gas pumped. A few providers charge a per-session fee or require a monthly or annual charging subscription. While many public chargers at businesses and parking lots remain free of cost to EV drivers, that is changing over time.

When you pay by the minute, charging cost is influenced by an EV battery’s state of charge, ambient temperature, and the size of the EV’s on board charger. Different size chargers can mean a big difference in the cost of a charge even though the same number of kW hours are delivered. For example, earlier Nissan LEAFs had a 3.6 kW (3.3 kW actual output) on board charger while later ones had an updated 6.6 kW (6.0 kW output) version. Thus, it takes almost twice as long to charge an earlier LEAF at double the expense than later ones, even though both have the same 30 kWh battery. Many EVs now come standard with a 6.6 or 7.2 kW charger. When considering buying or leasing an electric model, keep in mind that a more powerful on-board charger means quicker and potentially more cost-efficient charging.

It’s an interesting bit of science that while charging an electric vehicle, the rate of charge isn’t linear but rather decreases as a battery approaches full capacity. If an EV has a lower state of charge (SOC) at the beginning of a charging session, charging occurs at its maximum rate, such as 3.3 kW, 6.6 kW, 7.2 kW, and so on. As the battery approaches 100 percent SOC, charging can slow to a trickle. The last 20 percent of charge can sometimes take as long as the initial 80 percent. To be most cost efficient, it’s recommended to only charge to 80 percent full capacity when using a public charger, especially one that includes time-based pricing.

For a charging cost comparison, let’s look at charging an EV with a 40 kWh/100 mile rating and a 50 kW on board charger. At a Level 3 charging station it would take about 48 minutes to get an additional 100 miles of range and cost between $6.24 to $16.80, depending where you did the charging. With a 350 kW fast charger this would take about 7 minutes and cost between $1.82-$6.93 to add 100 miles. This compares to $10.00-$13.33 for a gasoline vehicle that gets 30 mpg and fuels up at $3.00 to $4.00 per gallon. This shows the need for fast charging when away from home and charging with time of use chargers, and more importantly, the need for pricing solely on a per kWh basis.

While kWh charging is fairer to the consumer, some companies prefer time-based charging because the longer customers are connected, the more profit is made. However, public charging could be moving from time-to-charge to the kWh charge model. This will put the energy cost of EV operation in line with that of gasoline vehicles where fueling cost is determined by the cost of a gallon of gasoline, not the time it takes to refuel. Clearly, this change is needed.

New rules in California will eventually ban public charging operators from billing by the minute and require the fairer billing by kWh. The ban will apply to new Level 2 chargers beginning in 2021, and to new DC fast chargers beginning in 2023. Chargers installed before 2021 can continue time-based billing until 2031 for Level 2 chargers or 2033 for DC fast chargers.

The new rules do not prohibit operators from charging overtime, connection, or parking fees, or fees for staying connected after reaching 100 percent SOC, providing they are disclosed. Electrify America already charges 40 cents per minute if your vehicle is not moved within the 10 minute grace period after your charging session is complete. It remains to be seen whether more states will follow California’s lead. Laws will have to be changed in about 20 states where only regulated utilities can presently sell electricity by the kWh.

Charging providers like Tesla and Brink presently charge by the kWh in states where it’s allowed. For example, Tesla charges $0.28 per kWh while Blink charges $0.39 to 0.79 per kWh, depending on location and user status. California regulations require Tesla and others to show the price per kWh and a running total of the energy delivered, just like a gas pump.

Other charging considerations can affect the actual long-term cost of operating an EV. These include lower charge pricing and discounts that come with subscriptions, free charging incentives that accompany a vehicle purchase (like the first 1000 kWh provided free or 100 kWh of free charging per month), or if a charger is shared with another user. For Teslas, free unlimited Supercharger access has often come with the purchase or lease of a new Tesla model.

While EV technology is now relatively mature, pricing electric vehicle use is evolving. Hopefully, competition and a bit of government regulation should ultimately make it as understandable as it is now for gasoline vehicles.

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.