Consider this scenario: It is 140 miles from Detroit to Kalamazoo, Mich. The average 2017 new car will make it on 5.6 gallons of gasoline and push 110 pounds of carbon dioxide into the air, according to EPA. A Chevrolet all-electric Bolt, on the other hand, will make the same trip without releasing a single carbon molecule from its battery-powered motors.
That seemingly straightforward scenario masks a lot of complexity.
The Electric Road Trip is calculating the energy and climate impact of its 6,000-mile journey, including the cost of fuel, the emissions of electrons versus gas, and how emissions change when we cross state lines. Every calculation of gas versus electric transport comes with caveats, exceptions and asterisks. We know that some people following the Electric Road Trip will want to understand those details.
So we've compiled this Q&A to answer queries readers may have.
How do you track the cost of charging? Often it seems to be zero.
The cost to the driver of charging an electric vehicle varies greatly. Often, filling a battery with electricity is significantly cheaper than filling a regular car with gas; occasionally, it costs more. In many cases, the driver pays nothing. The charging station operators pay the electric bill and offer the charging for free.
Some will cry foul and say this is an unequal comparison of the cost of electricity versus gasoline. Those people are right.
While the Electric Road Trip is underway, we are reporting the cost of charging sessions at face value, since it proved too complicated to crunch the numbers in real time. After the trip is over, we will take a look at the actual cost of the electrons and do a more rigorous comparison of the costs of electricity versus gasoline.
How do you compare the cost of gas?
At every charging stop, we open GasBuddy, a gas station locator app, and capture the lowest local price for a gallon of regular unleaded. We then divide the total miles of that leg of the trip by the most recent average miles per gallon figure for all U.S. passenger vehicles to get the trip's cost at the fuel pump.
Why is it hard to compare the emissions of an electric car to those of a gas car?
The climate advantage of electric vehicles over gasoline motors is a potentially crucial factor in shrinking new emissions of heat-trapping CO2 molecules in the atmosphere that are sickening the planet. But how big could that advantage be?
In planning the Electric Road Trip, we asked experts how to fairly compare the carbon benefit of electric vehicles we are driving against CO2 emissions from comparable passenger cars with gasoline-powered engines, if we'd driven those. The comparison, experts stress, does not stop with the vehicles' motors. It requires estimates of how much CO2 comes from the power plants generating the electricity that recharges our EVs every time we plug in.
No one right answer exists for what is a monumentally complex calculation, experts say.
Here are some of the complicating factors. CO2 emissions rates differ widely from coal-, natural gas-, and petroleum-fired power plants. Power from nuclear plants, wind farms and solar panels has no direct CO2 emissions at all. Furthermore, the power output from these generators changes constantly and flows across power lines without regard for state borders. (The analysts we spoke to considered these factors but not indirect costs, such as carbon emissions from mining and transporting coal, or producing and recycling EV batteries.)
OK, got it. So how do you compare emissions as you go from state to state?
We are comparing federal data on the 17 states we'll pass through on the Electric Road Trip. Experts recommended we use the U.S. Energy Information Administration's calculations of the annual carbon emissions for each state based on the output of power plants within its borders.
This isn't a perfect picture, but we focused on state numbers because state energy policies are key to the climate challenge.
The mix of power plants in individual states is as different as the states themselves. Coal-fired power plants produce the highest CO2 emissions per kilowatt-hour (kWh) of power. Natural gas-fired plants emit about half of coal's rate of CO2 emissions, versus zero for wind farms, solar panels, nuclear plants and hydropower dams. (A kilowatt is 1,000 watts. A kilowatt-hour refers to that amount of power delivered over an hour's time. An average American home consumed 867 kWh of electricity per month last year).
Kentucky's power plants, for example, using coal to generate three-quarters of the state's electricity output, emitted 1.9 pounds of CO2 per kWh of power in 2017. Illinois, whose power production includes no CO2 emissions from nuclear plants, discharged 0.85 pound. Washington state, drawing on hydropower from the Columbia and Snake river hydro dams for a majority of its power, produced 0.24 pound of CO2 per kWh.
How much CO2 does a conventional passenger car emit?
Let's go back to our trip from Detroit to Kalamazoo and look through the math.
Step 1: Start with the miles traveled per gallon of gasoline. We use EPA's average for all 2017 model year vehicles. EPA's number: 24.9 mpg. So the 140 miles for Detroit to Kalamazoo requires 140 miles divided by 24.9 mpg, or 5.6 gallons of gasoline.
Step 2: EPA reports that each gallon of gasoline burned in an average passenger vehicle engine releases 19.6 pounds of CO2.
Step 3: Multiply 5.6 gallons burned on the Detroit-Kalamazoo trip by 19.6 pounds of CO2 per gallon of gasoline consumed. The result, as we noted at the top: 110 pounds of CO2 out of the tailpipe for the trip.
How does an electric vehicle compare?
On the Electric Road Trip, we have the benefit of data from the car itself. But for now, let's stick with our hypothetical Detroit-Kalamazoo journey in a Chevy Bolt. We turn to EIA's report on Michigan.
Step 1: Start with how much battery power the EV drains per mile of driving. It varies with driving speed and highway terrain and can be read or calculated from information on an EV's dashboard screen. A figure the Bolt often achieves is 3.6 miles per kWh. That translates to 0.28 kWh of battery power used for each mile of driving on that leg.
Step 2: Multiply the 140 miles from Detroit to Kalamazoo by 0.28 to get 39 kWh of energy used on the trip.
Step 3: Average CO2 emissions from Michigan power plants equal 1.14 pounds CO2 per kWh of power generated, according to EIA's state-by-state data.
Step 4 (result): The Bolt CO2 emissions number for the Michigan leg is 39 kWh of energy used times the Michigan CO2 factor, 1.14, for a total of 44 pounds of CO2, compared with 110 pounds for the average passenger car. It's worth remembering that the EPA state averages for power plant CO2 emissions shrink as more renewable energy becomes available, as can happen overnight (when most EV home charging will occur) in wind-rich parts of the U.S.
Wow, that was complicated.
It sure was. Let us know what you think, and enjoy the periodic updates we'll provide on the energy and climate impacts of our ride, along with the other tough issues in electric vehicles that we'll try to straighten out.