|
Getting your Trinity Audio player ready...
|
There was an interesting discussion in the comments of a recent blog post, and I thought it would be fun to create a simple EV mileage calculator that uses local data about our power grid as well as the time of day you charge your car. So, here goes, in three simple steps.
Step One: Determine your kWh mileage (e.g., 3.5 miles/kWh).
First, estimate the number of miles you get per kWh. If you already know it, for example from your car’s data, then skip ahead to Step Two.
To get a rough estimate, take a look at this fueleconomy.gov page and find the model of the EV you are interested in. Look for the “kWh/100 miles” rating, which is a number between about 20 and 50.
Their estimate assumes you spend about 55% of your driving in city-like conditions, and about 45% in faster highway-like conditions. You can change that by using the blue “personalize” button at the top of the page. Set the time you spend in “stop and go” (city-like) conditions. The more time you spend at high speeds, the worse (higher) will be your kWh/100 miles number. (1) Other factors can also affect this number, such as how aggressively you drive, how hilly the terrain is, and how warm or cool the air is. Since it is relatively flat and temperate around here, it is okay to ignore those.
Where does the electric energy go? Interactive version at fueleconomy.gov
Once you have settled on the number, divide 100 by it to get miles per kWh. So 20 (kWh/100 miles) will become 5 (miles per kWh) and 40 will become 2.5.
Here are some examples:
– If you drive the base model 2019 Nissan Leaf, and 55% of your driving is “around town”, then you will be getting about 3.3 miles per kWh.
– If you drive the (much larger) 2019 Audi e-Tron, and 55% of your driving is “around town”, then you will be getting about 2.2 miles per kWh.
– If you drive the 2019 Hyundai Ioniq, and 90% of your driving is “around town”, then you will be getting an impressive 4.3 miles per kWh.
If you are interested, this writeup explains how the EPA derives those mileage estimate. Your mileage may vary 🙂
Step Two. Determine your charging factor (e.g., 0.5)
As was discussed in the post on the best time to charge your EV, the grid has more gas-generated power on it at certain times of day. As a rough estimate, if you always charge after work and/or at night, use 0.7 as your charging factor (the grid is roughly 70% power from gas). If you always charge midday, use 0.35 as your charging factor (the grid is roughly 35% power from gas). Use something in between if you use a mix of charging times.
Step Three. Do the math.
– Take your kWh mileage (from step 1) and multiply it by 120,429 (BTUs per gallon of gas, source).
– Divide that result by 7809 (BTUs per kWh for our gas-powered plants, source).
This is your miles per gallon if our grid were 100% gas.
– Divide that result by your charging factor (from step 2).
That number is your estimated miles per gallon of gas given the power you are using to charge your EV. (2)
For example, if your kWh mileage is a pretty good 3.7 and your charging factor is 0.4 (you nearly always charge at work during the day), then your estimated miles per gallon of gas is (3.7 * 120,429) / (7809 * 0.4) = 143 miles / gallon.
On the other hand, if your kWh mileage is a lower 3.0 and your charging factor is 0.7 (you charge in evenings and nights), then your estimated miles per gallon of gas is (3.0 * 120,429) / (7809 * 0.7) = 66 miles / gallon.
The EV mileage is better than that of an equivalent gas-powered car for three main reasons: (1) Our grid has a good amount of clean energy on it, which gives the EV “free” miles; (2) The gas is being burned in a relatively efficient power plant, rather than in a relatively inefficient gas engine; and (3) The electric car regenerates power from braking, which a traditional gas-powered car cannot do. (Hybrids also do this.)
It’s interesting to see the wide range of mileage you can get based on the size and weight of your car, where and how you drive it, and when you charge.
Share in the comments if you get any surprises, want to share your mileage, or have improvements to suggest for this mileage calculator. The biggest problem that I see is this calculator only looks at gas-powered electricity, whereas it would be better to look at emissions more generally. The other power sources do have some emissions, which would lower the overall “mileage” of the EV and decrease the impact of the charging time of day. For example, instead of using charging factors of 0.7 and 0.35, respectively, they might instead be more like 0.75 and 0.5. But I don’t have emissions data for each of type of power source, so that is just a guess.
Notes and References
0. This is not meant to be a full lifecycle emissions comparison between EVs and gas-powered cars. That would include additional sources, such as battery manufacture. For those who are interested, there is a recent excellent writeup that goes over some of the complexities in doing the comparison.
1. Electric vehicles get worse mileage at faster speeds, largely due to the loss of regenerative braking. See diagrams for City vs Highway on this page, for example. If you are interested, you can also compare the EV diagram to diagrams for hybrid and gas-powered cars.
2. The EPA provides a miles per gallon equivalent (MPGe) for each electric car, using a conversion factor of 33.7 kWh/gallon. That is based on the ratio of energy in a gallon of gas versus a kWh, and does not account for the inefficiency of power plants in converting gas to electric power. The conversion factor this calculator uses is 15.4 kWh/gallon (120,429 kWh/BTU / 7809 BTUs/gallon), and then on top of that it applies the charging factor, which accounts for the fact that only a fraction of the power on the grid comes from gas.
Current Climate Data (April 2019)
Global impacts, US impacts, CO2 metric, Climate dashboard (updated annually)
Comment Guidelines
I hope that your contributions will be an important part of this blog. To keep the discussion productive, please adhere to these guidelines, or your comment may be moderated:
– Avoid disrespectful, disparaging, snide, angry, or ad hominem comments.
– Stay fact-based, and provide references (esp links) as helpful.
– Stay on topic.
– In general, maintain this as a welcoming space for all readers.
.