"Pavel Gilman, vice president of sales and marketing at Nivalis, breaks down where those savings come from: roughly 30 to 35 percent from the electric axle during braking and deceleration, 11 to 15 percent from the rooftop solar panels, and the remainder (roughly half) from grid charging during parking stops."

11..15% Is not the bulk of the energy. But it's significantly more than "negligible".

Note that this trailer could be expected to be on the road most of the time. Which may not even be the common case: demand for goods varies a lot, so I'd expect that a lot of logistics companies aren't fully booked all the time. Maybe have a few drivers/trucks on the road, and a bunch of trailers on a parking lot doing nothing. In that scenario: need a trailer? Use whichever one is charged up furthest, while at a loading dock top it up (grid power/in-building storage) to whatever is needed to complete the trip, off it goes.

Also for US-based naysayers: Europe is a very different environment than the US. Most long hauls aren't that long, and there's a lot of [industrial zone local distribution center] <-> inner city traffic. Read: lots of start-stop, low speed cornering & wait time.