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Solar on the roof is most likely rather insignificant and will also just add weight.
An ISO container that could fit on a truck trailer is typically 40'x8'. Solar panels are, on the sunniest of days, making 10-14 watts per square foot. This gives us:
40x8=320sqft assuming panels with no borders.
Assuming 14 watts per sqft (optimal conditions) we'd end up with 4,480 watts. Assuming 100% efficiency converting this into usable motion, we'd get 6 BHP. A normal semi truck has 350-600HP, so this is a drop in the bucket compared to their normal output power. As far as how much of that they use while cruising, I'd wager that it's much more than the 1-2% boost which solar-powered electric would provide (again remember that my calculations assume cloudless sky, no dust on panels, no weight or aerodynamic penalty, no maintenance costs, 100% conversion efficiency, etc).
They reason you don't see them is because it's not worth it. The aero mods under and behind the trailer on the other hand ARE worth it, and this claims that a specific version of a "trailer tail" gives a 6.6% boost to fuel economy at 65MPH: http://realtruckdriver.com/what-do-i-think-about-semi-truck-...
The truck logistics industry has been struggling with this problem for years. How do we increase fuel efficiency from the days of 90s Caterpillar engines dragging 80K lbs at 4.3 mpg while simultaneously decreasing emissions while simultaneously figuring out how to make the truck itself lighter and able to move more cargo in a single load?
Autonomous electric trucks would improve "fuel" economy and minimize downtime caused by the limits of human endurance, but at what cost? How many miles of range would the batteries have to have to compensate for the slower cruising speed and additional stops needed to recharge them? How much DOT downtime has to be eliminated to make up for being able to pull a few thousand pounds less on each trip, owing to the massive number of LiOn batteries scattered around the truck and trailer?
When I worked at Penske Truck Leasing in the late aughts, I'd have fascinating conversations with truck drivers on the math they were doing to optimize for fuel economy and load capacity. The switch one of Penske's clients made in changing their fleet to DEF Volvos with 240 gallons of diesel capacity compared to the classic Detroit Diesel Freightliner Centuries with dual 150-gallon tanks caused many an angry tirade.
Many of the drivers were constantly juggling juuuuuust enough fuel to make it to the destination and back; the trailer they were headed to pick up was so heavy they couldn't have more than ~200 gallons of fuel or they'd be over 80K and fined accordingly at the scales. Good luck making that 1200 mile round trip on your 200 gallons, driver.
I'm not sure trucks are the answer. We'd need too much overhauling of the current system to make it worthwhile. Without increasing weight loads, which destroys our infrastructure even faster, as well as rethinking how trailers are loaded and unloaded to maximize the 24/7 automated nature of autonomous fleets... We're a long way off.
And, anyways, aren't cargo ships and trains basically already electric and their massive diesel engines are just powering the electric motors? No weight limits or problems caused by slow cruising speeds in those applications, that I'm aware of.
It actually shows that neither of you are paying attention. Musks article did not say anything about putting solar panels on the roof of trucks. He only talks about solar panels on the roof of buildings.
