I run 29x2.0 Schwalbe Marathon Almotion on my e-bike (Cross Current.) They offer them in 26x2.15. They are the lowest rolling resistance touring tire tested by these guys. They have the same 3mm Greenguard as the Schwalbe Marathons. Heavy tires but that's not really an issue on an e-bike. If your frame has clearance I'd go as wide as possible and drop the pressure down. You start to feel every bump in the road when you're cruising well over 20mph on a rigid aluminum frame. I'd love to run the 2.35" Schwalbe G-One Speed if my frame had enough clearance. I have a suspension fork and Body Float seat post on my e-bike and I still appreciate more compliant tires.