> Could they not install solar panels on the roof and some powerwall size batteries?
Nope, the sort of batteries you need are more like "Dinorwig Power Station" Find a large mountain. Bore out the centre and when you've got excess power pump water up to the top. When you need to charge all those delivery vehicles start letting the water pour back down the hill.
Pity the article doesn't give any details of the power draw needed. Telsa's site says you'll need 16.5kW for 2 hours to go 100miles. They've got 49,000 vehicles, assuming 2 hours each per day that's going to be a lot of power. Telsa's speal about powerwall talks about an installation in Hawaii which is 13MW and requires 55,000 solar panels. RM would need at least 5 of these just to power their fleet and we don't get anything like the sun that they get in Hawaii.