The need to reduce carbon emissions has led automakers to make major investments in electric cars, but sales remain low. Globally, electric cars made up around 2 percent of new-car sales in 2019. Jason Fenske of Engineering Explained offers an explanation for why EV adoption may be stagnating.

The main issue, according to Fenske, is that batteries simply aren't as energy dense as gasoline. That means more space is required to hold a given amount of energy with batteries than with good old dinosaur juice. A gallon of gasoline is equivalent to 33.7 kilowatt-hours of energy, according to Fenske. So that one gallon contains more energy than the entire battery pack of a first-generation Nissan Leaf.

However, electric cars are generally more efficient than gasoline cars, Fenske noted. The 2ZR-FXE engine used in the Toyota Prius is one of the most efficient internal-combustion engines in production, at around 40 percent, according to Toyota. But most electric motors can consistently operate at 90 percent efficiency, according to Fenske.

But the efficiency difference is largely negated by other factors, Fenske said. Differences in driving conditions and cold ambient conditions can lower the efficiency of electric cars, he said.

The extra weight and complexity of a battery pack compared to a fuel tank is also an issue. Getting sufficient range from an electric car involves adding more battery cells, which take up space and increase a car's curb weight, Fenske said.

This isn't as much of an issue for electric passenger cars as it is for other types of electric vehicles, according to Fenske. The weight of semi trucks, for example, is limited by regulations. So getting enough range out of an electric semi truck could severely limit the amount of cargo it can carry, Fenske said.

That hasn't stopped companies from trying to develop electric semi trucks. Tesla has said its Semi will have a maximum range of 500 miles, and the truck has attracted interest from companies like Budweiser and Walmart. It's worth noting that early electric semi trucks will likely be used on shorter runs between dedicated terminals, ensuring they're never too far from a charging station.

Energy density could also impact the many electric pickup trucks currently in development. Towing inherently lowers efficiency, but that generally isn't a problem for gasoline and diesel pickup trucks. Even if they're getting worse fuel economy by towing, large fuel tanks ensure drivers won't be inconvenienced. In contrast, towing with an electric vehicle can sharply decrease range.