Power to weight doesn’t matter as we are talking about using a solar panel instead of a roof. There’s no added weight. The car will already have inbuilt inverters so the only real weight add is the wiring. But also, this is a postal vehicle which will have large swings in weight anyways. A couple of extra pounds doesn’t make a difference here.
Further, this isn’t a car, which has a much smaller surface area. These things have about 10 square meters of flat roof. That’s a peak output of ~3kW. (realistically, probably closer to 1.5kW average throughout a day) which translates into 12kWh of charge in any given day. Roughly 10% of the battery capacity could be restored daily.
For large vehicles, like delivery vehicles and busses, the math on making the roof out solar panels instead of steel changes.
A roof only needs to be a thin piece of sheet metal, weighing somewhere in the neighborhood of 1-2lbs per square foot
Most solar panels are going to weigh somewhere in the neighborhood of 2-4lbs per square foot. So is likely the panel would weigh 2-4 times as much as just a plain metal roof, plus possibly a metal roof under it and/or additional framing to attach the panels to, so power to weight does absolutely come into play.
That comes in at an additional 400lbs on a vehicle that weighs 6,670 lbs if you assume the maximal weight of the panels and the need for a metal roof anyways. That extra 6% weight just doesn’t matter. 6% losses range for 12% free charging seems like a worthy tradeoff to me. (and again, this is the maximal value).
