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Comment by SV_BubbleTime | Hacker News Reader

Does anyone with an EV that has a KW/h gauge actually believe that a semi will run on the highway at 2KW/h?

To people without EVs, at 2KW in your car is idling forward from a stop. When you press the gas at a normal street at a normal start, you’ll be at 10-18KW/h until you are up to speed. A hard pull is 24KW/h, and on the highway I would assume 4-6KW/h in a light unloaded vehicle. So maybe they had some new amazing Tesla transmissions and Tesla low friction tires too?

EDIT: yes, obviously I’m talking about KWh instantaneous. So /second/minute/hour for the short frame until the next update. If you were to hold this power for the end of time you would use xx KW in an hour. Which is useful for the feeling of torque I described.

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You have manhandled your units as badly as is possible.

Model S cruises at approximately 300Wh/mi; Tesla is claiming 2kWh/mi (approximately 6.6x the power consumption) for the Semi.

Diesel cars are rated at approximately 35mpg and diesel semi trucks at approximately 5mpg, a similar 7x factor.

So, yes in fact as the driver of an EV with an energy meter, I do 100% believe the 2kWh/mile number.

What I'm actually skeptical of and where I believe Tesla is burying the bad numbers is with the $/cargo ton/mile rating. Carrying an extra 8 tons of battery is going to limit the types of loads where an electric semi mikes sense.

> Carrying an extra 8 tons of battery is going to limit the types of loads where an electric semi mikes sense.

Tesla claims the Semi is only 1/2 ton heavier than a typical tractor, not sure where you got the 8 tons figure from…

My 2015 Tesla S 70D does roughly between 260 Wh/m and 370 Wh/m depending on terrain, temperature, precipitation, wind, speed, and driving style. I have no idea what your units mean. A hard acceleration is 250 kW. The average power at a constant 60 mph is about 18 kW.

2 kWh/m seems plausible for level terrain for a larger vehicle that is presumably optimized for low rolling resistance. It sounds a little optimistic but not by more than a factor of two.

So my answer is yes I can believe it or something close.

You are right to be confused about that claim, but it because the units of the claim have been mixed up. The actual claim is 2 kWh/mile. That is a measure of energy per distance, kind of like gallons per mile.

If a truck is using 2 kWh/mile while traveling down highway at 70 miles per hour, its power draw will be 140 kW -- that is the one to compare to your 2 kW idling or 24 kW hard pull.

My EV uses 0.2 to 0.3 kWh/mile, so 2 kWh/mile for a truck seems plausible to me.

I think there's a unit issue here. The likely speed of a semi is ~60mph, which conveniently is 1mile/min. So even if you're using 20kW (>25hp) continuously or more likely 120kW (>160hp) you'll travel 60 miles in a hour.

So 2kWh/mi = 120kWh/60mi.

My Tesla Y gets 3 mi/kW. They said a normal diesel truck gets 6 mpg, which is about one fifth a normal passenger car on the highway at 30 mpg. So 2 kW per mile doesn’t sound unreasonable.

You’re comparing KW/h to miles per KW - it doesn’t work that way. At 4-6KW/h on the highway, you’re doing 70+mph, so that’s 6KW/70mi, which is <0.1kw/mile.

I am guessing regen is a big factor

kW/h is not an unit you want to use. Just drop the /h. And math makes sense.

But yes 2 kW is very low power. Around 2.7 horse power. Which is very little at highway speed. Specially with drag of any semi-truck.

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