AP NEWS

Fuel Economy Requires Advanced Math Degree

April 6, 2019

Q:We own a 2018 BMW X5 40e gasoline-electric plug-in hybrid. According to BMW, using the 3.5 kWh charger on a standard household outlet (which I have read uses 1,440 watts) takes three hours and 45 minutes to charge the car from a depleted state. With that charge, I get 14-19 miles of all electric power before the car turns into a regular hybrid. Where I live in (expensive) Rhode Island, electricity costs 22.67 cents per kWh, including taxes and fees. So I’m trying to do some calculations. Would the correct formula be: The watts (1,440 per kWh) divided by 1,000, times the hours (3.75), times the cost per kilowatt hour (.2267)? Would that tell me how much it’s costing me to charge my battery? If so, this equals about $1.23 per charge. That would be great for a range of 14-19 miles on a charge, less than half the cost of a gallon of 91 octane fuel. The alternative would be to multiply the kilowatt hours (3.5) times the hours (3.75), times the cost per kilowatt hour (.2267). That would mean it’s costing me $3.17 a charge, which would be awful, because it would be more expensive than premium gasoline. Is one of my formulas right? Thanks. — RICK A: You know what’s worse than spending an afternoon screwing up a complicated math problem, Rick? Finally figuring it out and realizing that your car costs more to run on electricity than on gasoline. Forget your formulas, Rick. The easiest way to calculate your cost per mile is to start by looking up how many kWh (kilowatt hours) your car requires to travel 100 miles. That’s a standard measure for electric cars, and you can find it by going to the EPA’s consumer website, www.fueleconomy.gov. You bought a big, heavy SUV, and it takes 59 kWh to push that BMW X5 100 miles. That’s on the high side. Just for comparison, the Chevy Bolt requires 28 kWh to go 100 miles. Anyway, you then multiply your car’s kWh/100 miles (59) times your cost per kW (0.2267, which is on your electric bill), and you find out that it costs you $13.38 to run your car 100 miles on electricity. If you divide that by 100, you get your cost per mile, which is about 13.4 cents. To compare that to gasoline, take the local cost of a gallon of gas (the X5 requires premium fuel). Let’s call that 3 bucks a gallon. According to the EPA, your X5 hybrid gets 24 mpg on gas, so you’d need 4.17 gallons to go 100 miles. Multiply the number of gallons times the price per gallon, and you learn that you need $12.51 worth of fuel to go 100 miles, or 12.5 cents a mile. So it costs you a penny more per mile to run on electricity. Doesn’t that frost your shorts? The problem, as you seem to understand in your letter, is that the cost of electricity is pretty high in your state, and the price of gasoline is pretty low. In many parts of the country, running a car on electricity costs quite a bit less than running on gasoline. But it’s worth doing the calculations before you buy.