Tesla Semi Regen Roundtrip Efficiency is ~95%. Battery Size is 900kWh.

Tesla shared a screen of the energy graph for a 500 mile trip. What can we learn from it?

TL;DR: Based on the 500 mile energy graph, the Semi regen efficiency is insane. Roundtrip is ~95%, and one way is ~97%. The net cost of climbing/descending 1000ft elevation is 1.5kWh, less than the cost of driving one mile.

In the graph that Tesla shared the blue line shows the state of charge of the battery over the course of the drive. The gray area is the elevation profile of the trip. At constant speed and constant grade (e.g. everything is flat because we start and end at about sea level) we would expect a straight blue line. A cursory glance shows that pretty much all the energy used for the various climbs is regained on the way down. The real world graph (with hills) always goes back to the straight line graph. So the efficiency is pretty good. But how good.

Annotated Energy graph of Tesla's 500 mile run.
Annotated Energy graph of Tesla’s 500 mile run.

Battery size according to the Grapevine Tells Us

First we need to get a better estimate of the battery size. Let’s look at just the Grapevine (Tejon Pass). The elevation difference from the lowest point just before to the pass is about 3900ft over 25 miles. The battery percentage drop over that period is ~18.5%. Driving 25 miles at 1.7kWh/mile is about 43kWh. The potential energy gained is about 120kWh. We use 97% efficiency for the climbing part. This leads us to a battery size of ~900kWh (calculator) (90% efficiency would make that a 950kWh battery).

End to End Efficiency

Google Earth View of Tesla's 500 mile run.
Google Earth View of Tesla’s 500 mile run. What’s of particular interest here is the ~14,400 total elevation gain.

The total elevation gain on the 500 mile run is about 14,400ft, and it requires about 445kWh to lift 82,000lbs that much (calculator). With 100% efficiency, we would get those same 445kWh back. But we have losses both going up and coming down (regen). If for simplicity the efficiency going up and down is the same, our roundtrip efficiency will be the square of the one way efficiency.

How much are the losses. The green line is set to match the slope of the mostly flat with just minimal elevation gain between mile posts 200 and 250. If we extrapolate that angle from the starting point to the end, we can see the difference between the green line and the final dot is about 2.5% of a battery. The run used 93% of the battery. This leads to a round trip efficiency of 95% (calculator). This is insane. One way efficiency would be over 97%.

Let’s double check: How stable is this calculation. We assume 82,000lbs. If the truck was 60,000lbs total, and we used 3%, round trip would be 92%. Still impressive. You can play with the numbers in the calculator. I’ll go with 97% for my Tesla Semi Numbers table.

What does that mean in practice? It means that the nest cost to climb and descend 1000ft with a fully loaded Semi truck is about 1.5kWh. The same as the cost of driving one mile!

Last Updated on December 10, 2022.