The low cost of fueling an electric vehicle is often cited as its chief benefit. Plug-in car advocates say that battery-powered cars cost about $0.02 per mile to run—that’s roughly one-sixth the cost of powering a moderately fuel-efficient gasoline-powered car. But just how accurate is that number?
Electric car drivers don’t typically monitor their EV’s electricity use—their home and car’s energy consumption often get lumped together. Whether metered separately or not, the price of electricity can vary depending on the time of day, time of year, and the overall monthly electricity consumption, making quantifying EV electricity use even more complicated. All details, such as your local rates and type of electric car, could shift the numbers one way or the other.
It would be a fool’s errand to try nailing down a single price for all EV drivers across the country—or even in one town. Instead, I’ll focus on what I paid during two specific months in 2012—February and May. The “test case” is my all-electric 2012 Nissan Leaf plugged into Pacific Gas and Electric’s (PG&E) grid in the San Francisco Bay Area. A 3.3 kW grid-tied, net-metered PV system offsets some of my household’s electricity and provides some charging electricity for the Leaf.
PG&E offers special EV residential electricity rates, and with its seasonal and usage tiers, the price for 1 kWh of electricity (in the high season—summer) ranges from $0.037 from midnight to 7 a.m. to as much as $0.54 per kWh—almost 15 times as much—during a summer afternoon.
I determined my EV’s charging usage through reports provided by Blink, the charging network that tracks how much electricity flows through my home charger (which they document as part of The EV Project, supported by the U.S. Department of Energy—see sidebar).
In February, I used 385 kWh to recharge the Leaf—out of the total of 954 kWh I purchased from PG&E. In May, I bought a lot less electricity (461 kWh), since my PV system was generating more electricity, but the EV still used almost as much juice—342 kWh.
Are PG&E’s current EV electricity rates confusing? Extremely. If I begin a four-hour charge at 10 p.m. in the winter, the first two hours of charging cost $0.09 per kWh. But at midnight, the price is cut in half to about $0.045 per kWh. Let’s assume that charging energy use is steady for the entire four hours, even though the energy flows a little faster during the first few hours and tapers off as the battery fills up. And for simplicity, let’s say that I charge exactly the same amount every day of the month, since averaging helps calculate the impact of a tiered pricing structure.
In February, for the first 10 days of usage, I paid $0.046 per kWh. On the eleventh day, I reached the baseline for Tier 1, which then pushed my rate up 40%, to $0.0645. By May 19, my rate for daytime peak electricity jumped to a whopping $0.54 per kWh. Utilities rightly discourage EV charging and other energy use during the day in the summer, when most air conditioners are running.
In February 2012, I used 385.12 kWh to recharge my Leaf. Of those, 243.31 kWh were during off-peak hours, costing $28.93. The other 142 kWh were during partial-peak hours—weekday mornings. For the first 49 kWh of those partial-peak 142 kWh, I spent $4.82. But because of tiered price structure, the final 44 kWh of the month cost $14.83—more than three times the cost for fewer kWh. The total for partial-peak charging was $30.65. The combined off-peak and partial-peak for February’s EV electricity bill was $59.58. That’s an average rate (across all times and tiers) of $0.1547 per kWh.
According to my Leaf’s onboard stats, I am a moderately efficient driver at 3.3 miles per kWh. (EV drivers who are light on acceleration and good with coasting can get 4+ miles per kWh.) But for me, those 385 kWh provided 1,270 miles of travel.
If I had been driving a 40 mpg gasoline-powered car, the cost for those 1,270 miles would have been $127, at $4 per gallon. So the cost savings in the electric car in February was $67.41. If all of my charging would have been during off-peak hours, I could have added another $14 to that savings.