Public charging has been provided for free at many retail locations and parking garages as a way to encourage buying an EV, or to enhance an establishment’s environmental image. Now, pay-per-use chargers are increasingly the norm. But at what cost?

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.