Making the right comparison

The answer you get depends on the question you ask.

One of the claims made in favor of electric scooters, those trendy objects now appearing in many cities, is that they’re more eco-friendly than cars.  That is, they don’t emit the pollutants that so obviously enter the air around major highways.  There is no exhaust, so nothing comes out, right?

Well, there’s more to it than that.  Let’s take, as a measure of emissions, carbon per mile.  A car getting 15 miles per gallon uses 30.2 cubic centimeters of fuel per mile, and if that’s composed of octane (which is a reasonable simplification) it’s putting out 22.8 grams of carbon per mile, mostly in the form of carbon dioxide.

The scooter itself puts out nothing.  But you have to charge the scooter.  Suppose you use power from Dominion Energy Virginia, as one does in this part of the world.  Their website gives a figure of 0.273 metric tons of carbon emitted per megawatt-hour generated.  Using the numbers given for one electric scooter, that its 473 watt-hour battery is good for 30 miles, one eventually gets to 4.28 grams of carbon per mile.

So the scooter wins still.  But we’re not quite comparing like with like; there are losses in transmission all along the way, and we think it’s reasonable to assume they could add up to 50% between the generating plant and the scooter battery, giving something like 8 or 9 grams per mile.  And a modern low-emission car might get double the mileage assumed above, which makes its figure about 11 grams per mile.  Under the rather rough assumptions we’ve made, we can say that their emissions are comparable.

But we need to ask: what have we compared?  We’ve moved one scooter and one car one mile each.  If you have two people in the car the mileage will be almost exactly the same, while you’ll need two scooters.  (It is against the rules for two people to ride on one scooter, though we’ve seen it done.  It certainly has a significant effect on the efficiency of the vehicle.)  Similarly, if you have anything heavy to carry, the scooter will have some trouble (at least) while the car will have little or none.

It looks probable, though, that for moving one person a mile or two, the scooter actually has a lower level of emission compared to a mostly-empty car.  We wonder how often people actually drive a mile, alone, in a car.  Doing it in Old Town Alexandria, with its maze of narrow streets and stop signs, is a recipe for frustration; we always walk that distance.  Some people, of course, are unable to walk that far; but we’re not sure that many of those would be physically able to ride a scooter, either, which takes some dexterity and sustained standing.  The scooter will get there sooner, which may be important to you, though it’s generally irrelevant to any discussion of eco-friendliness.  For going a few miles many people ride bicycles, which have no emissions at all (at the level we’re studying) and go quite as fast as scooters.

Then we can ask more questions about the emissions.  Car emissions pass through a catalytic converter, which does away with much of the most harmful chemicals.  Few fossil-fuel power plants do the same.

So to answer the question: “How eco-friendly is the electric scooter?” we have to decide what kind of journeys to compare, and exactly how.  The answer is not as obvious as it first appears.