Submitted by WannaBelTGuy t3_zz8gia in explainlikeimfive
Context: I sell cars and currently have a Nissan Leaf. A customer had asked if I knew why it had 121city/91highway MPG listed if it was a fully electric vehicle. This confused me too and it’s apparently a common thing? Thanks.
Okay, so it’s essentially how much the battery is used over that distance? Like how many miles would be used per gallon.
Yup.
In most of the world, fuel consumption is litres per kilometre, which maps to battery consumption in a more logical way. The MPG rating is often just the l/km rating converted.
Awesome! Thank you!
Its a measure of efficiency to allow people to compare the energy consumption, they're rated in MPGe (Miles Per Gallon equivalent)
Why list it as Miles per Gallon instead of kWh/mile or miles/kWh? Because they did some surveying and determined people don't know what a kWh means in terms of their car and found MPG less confusing and consistent with the existing markings
121/91 MPGe city means the Leaf used 0.278 kWh/mile on the city driving test and 0.370 kWh/mile on the highway test
It doesn't directly compare to the costs/CO2 emissions of a gasoline powered vehicle because it doesn't deal with the upstream power generator efficiency but its a start and allows for comparisons of efficiency between electric cars beyond just the stated range which is heavily influenced by overall battery capacity
Thank you! I looked like an idiot out there and completely lost the customer.
How else are you going to compare the efficiency to an ICE vehicle?
It doesn't seem like a useful comparison to me to be honest. On an ICE vehicle I can pretty easily use that number to get an idea of how expensive the car is to drive. On an electric only vehicle, that number doesn't really tell me anything useful about the car. Maybe it makes it look more "energy efficient" but that's not the important thing to me. I want to know how much it costs to drive.
It would make more sense to state what % battery loss is used per hour or city/highway driving, or total time you can drive for city/highway.
The Leaf advertising 121 MPG means nothing, as I don’t know how many “gallons” a full charge has. My car advertises like 36 MPG city, and with a 12gal tank I know I can drive ~430 miles.
This is a great point. If they're going to quote miles per gallon as a fuel efficiency figure then they also need to prominently display the battery capacity in gallons. Multiplying the two numbers should give you the range in miles.
At some point it will make sense to change to kW-hours but not while the great majority of cars still run on gas. It's the same with light bulbs where people know how bright a 60W incandescent bulb is but still don't know that that's about 800 lumens. Some LED bulbs are marketed with incandescent-equivalent wattage more prominent that actual wattage.
You are conflating range (time or distance) vs efficiency. First, % battery loss per time would be an unnecessarily complex metric, you would just put the time. That does not tell you how efficient the car is as battery size is not held steady. In terms of a gas car, a Hummer could have a longer time until you need to fill up, but it is obviously not as efficient as a Prius. MPGe while not necessarily the best metric, does provide an efficency metric. It also has a rough correlation to the MPG metric used for years in the US.
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It's a way to easily compare vehicle efficiency. Personally, I used $/mi to compare my leaf's operational costs and the ice vehicle it replaced. I think Nissan's marketing called it eMiles at the time.
I'm glad you asked this question, I didn't know there was a rating, and this is super interesting to me. I had no idea that EVs were that much more efficient at converting energy, and had assumed that the benefits were something of a wash if their supporting powergrid was using coal or petroleum to generate the electricity to charge the car.
Doing a little research, it looks like the range of efficiency is 120 MPGe for a Tesla Model 3 (the best), and 68 MPGe for a Ford F-150 Lightning (the worst).
That tells me that the absolute worst electron guzzlers are still better than the absolute best internal combustion vehicles (Hundai Ioniq at 59 MPG for 2022).
Obviously there are additional environmental concerns with the original manufacture of electric vehicles, especially in coal burning countries like China, but if your electric grid is backed up by wind, solar, or hydro, this seems like a cowabunga slam-dunk.
Am I forgetting something, like power lost in transmission through a grid?
TLDR: Electric vehicles get way more miles per unit of energy, but not per unit of mass/volume of energy storage.
Internal combustion engines are fundamentally pretty inefficient-only about 30% of the energy they use goes to actually moving the car forward. This is related to how energy is extracted from the gasoline (research Carnot efficiency if you want to know more about the theory of how heat engines like we have in our cars work). The reason we can still make practical cars this way is that gasoline has a lot of energy packed into a relatively small mass/volume.
Electric motors have much higher efficiencies-around 90% of the energy they use goes to moving the car forward. However, the range is limited by the capacity of the battery, and the energy to mass/volume ratio for a battery is way less good than gasoline. We've only gotten the battery technology good enough to compete with gasoline on range relatively recently (especially if you consider the difference in recharge vs refill time).
It’s MPGe (miles per gallon equivalent), and it was invented so that consumers could (sort of) compare the efficiency of electric cars and non-electric cars. Unfortunately, it’s not intuitive in any way.
The US Environmental Protection Agency determined that 33.7 kWh of electricity was equivalent to 1 gallon of gas. So, if a car goes 4 miles on 1 kWh of electricity, that’s equivalent to 33.7 x 4 = 134.8 MPGe
The 33.7 kWh per gallon assumes 100% efficiency in conversion of the chemical energy of combustion in a gallon of gas into mechanical energy. However, a typical internal combustion engine is typically about 25% efficient (and electric motors close to 98%). As a result, it’s a pretty dubious way to compare EVs and non-EVs. It’s an OK way to compare EV to EV, though miles per kWh is more meaningful because it can be easily used to estimate the cost of operating the car.
Em_Adespoton t1_j2a27o0 wrote
It’s an equivalence thing so you can compare the efficiency of ICE, BEV and PHEV vehicles.
They just compare energy consumption and convert it to if that energy was from gasoline.