Do the math: Tesla’s truck numbers just don’t add up
Tesla has finally unveiled its much-promised big rig. And with not a little fanfare, especially considering that said semi is claimed to have a range of 500 miles (800 kilometres!) and, more importantly — at least for fleets seriously considering an all-electric 18-wheeled future — is able to recharge 400 of those miles (640 km) in just 30 minutes. So the question is, has Elon Musk really reinvented the electric vehicle yet again? Or are his latest claims of reimagining heavy-duty transport just more of his Madoffian fantasy?
To find out, Motor Mouth broke out the old calculator — OK, my new iPhone 8; one must be au courante, after all — and used those few numbers Musk was willing to share to try to shed some light on some claims that would seem to be the death knell of the diesel truck. The most important calculation, of course, is what Musk’s semi has under the hood — battery-wise that is, not engine — since what we all want to know is how many kilowatt-hours Musk imagines it takes to get a truck that may weigh as much as 80,000 pounds fully loaded to travel 500 miles on a single charge. Plugging what we know — 30 minutes of recharging time and the fact that the biggest recharger available is 600 kilowatts — into some fairly simple formulae and we arrive at a number that says Musk estimates his sleek semi will require about 300 kilowatt-hours to travel 400 miles.
Now, here’s where those numbers go just slightly awry. Musk’s sleek Model S — a bit of a porker but aerodynamically efficient nonetheless — needs just a hair under 0.33 kilowatthours to travel one mile. So, if it, too, were to claim a 400-mile range, it would need about a 135 kW-h battery. Now, I am pretty sure that it doesn’t take a degree in rocket science to figure out that one of those calculations — a Model S needing 135 kW-hr to travel 400 miles or a full-sized eighteen-wheeler requiring just 300 kW-hr to do the same — is a little wonky. The truck is, after all, about 15 times heavier and probably has at least three times the aerodynamic resistance.
For those needing a little more arithmetic backup, consider the following: A current, fully loaded 18-wheeler similar in shape and size to Tesla’s big rig can consume anywhere between 40 litres and 50 litres of diesel fuel per 100 km while cruising at about 100 km/h. By way of comparison, an Audi A7 — similar in size and shape to a Model S, but also diesel powered — consumes about six L/100 km. And that’s with your humble Motor Mouth hogging the fast lane at about 120 km/h. Simple math, then, says that a ginormous truck consumes somewhere in the vicinity of six to eight times more fuel to cover the same distance than the itty, bitty car. As further comparator, big rigs can use up 10 times as much horsepower to cruise at 100 km/h as a car, but we’ll stick with the more conservative estimate of six to eight for our calculations.
If we use the median of those figures and assume that Musk’s truck requires eight times the battery as his Model S to cover the same distance, then, that 500-mile range he claims requires somewhere around 1,000 kW h to power. At current prices, the batteries alone could cost as much as US$200,000, a figure that jives (roughly) with a recent Carnegie Mellon study on electric semi trucks that determined that “a 300 milecapable battery pack costs about $200,000.” An entire diesel truck, by way of comparison, costs about US$120,000. That same study also estimates that the battery required for a longdistance big rig could weigh as much as 22 tons. In other words, according to the study, the truck’s battery is heavier than its payload.
More dramatically, plugging those numbers — 1,000 kW-h rechargeable in 30 minutes — into those same basic recharging calculations tells us a 2 MW (yes, two megawatts!) charger would be required to replenish the new Tesla 18-wheeler in the time Musk claims. That, as they say, is a game changer, because the 0.6 MW unit I mentioned earlier is so powerful it needs to be fully automated, is about the size of a small gas station kiosk and costs in the range of half a million bucks.
And, lest you think I am being overly harsh with my estimations, that aforementioned Carnegie Mellon study (Evaluating the Potential of Platooning in Lowering the Required Performance metrics of Li-ion Batteries to Enable Practical Electric Semi-Trucks) estimated that 1,000 kW-h would only generate 300 miles of range.So, in fact, Tesla’s proposed Megacharger might have to be larger than two megawatts if Tesla wants to recharge 400 miles in just 30 minutes.
Unlike previous Motor Mouth columns regarding Musk’s claims, I will pass no judgment on whether these latest pronouncements are feasible or outrageous. I am, frankly, tired of his acolytes portraying me as anti-electric and, more insulting, anti-progress. I will, instead, simply offer these calculations as a starting point for discussion. Make of them what you will.