From quick search electric car uses 0.15-0.2kWh per kilometer. That's amount of energy you get after delivering 200 Watts for an hour. Let's say you have 2m2 of roof (pretty generous), of very efficient panels in your driving in absolute optimal conditions when they are giving you full power (that's around 200W/m2). No loses on the inverter and such.
So you need to be going at the speed of 2km/h. Well, that's actually not that bad for a postal service. It's assuming absolute optimal conditions, but energy usage is for a typical electric car.
I doubt it would be financially net positive or carbon negative, but the idea is not as crazy as I thought assuming final delivery in medium density sunny urban areas.
I don't think the urban areas would work/are needed. Right now all mail is delivered by foot and likely faster than you could deliver with any vehicle, lots are 25-30feet wide so the density makes walking the best method. Where I do see it happening is out in the suburban subdivisions where people don't even have personal mailboxes, there's usually a cluster of boxes in the middle of the block for all the residents. So deliveries would be every 400m or so, electric would be perfect.
AC is pretty power hungry; solar panels on the roof of an average van (best case) aren't going to make much of a dent. I find estimates of 10-20 miles per day of range in the best cases.
At current prices, my sense is that solar panels on vehicles are marketing, not materially contributing to the energy budget. Nothing wrong with that but I wouldn't like to spend my feature money on the option.
