I hope you don't mind, but I moved all your threads about this project into your original one to keep all the project info and discussion in one place, so you don't have some people helping you in one and some in others, each asking questions already answered in other threads they don't know about, and to help you find all the answers to make building it easier.
What you will need depends on exactly what you want it to do under what specific conditions. Some of that's already been discussed above (conditions, weight, speed), but not everything was answered yet (range, acceleration, etc). If you take the specifics of the job you need it to do and put them into the ebikes.ca motor and/or trip simulators, you can play with different systems to see if they will do the job, and what kind of power levels you will need to do the job under your conditions.
As a guesstimate from my own SB Cruiser usage, I'd say that 2kW+ is likely to be useful, 800-1200W is probably minimum required, 4kW+ will be much better performance on acceleration / headwinds.
If you don't need quick acceleration, and don't mind taking up to a quarter mile or half mile or more to reach 20mph, a 1000W hubmotor will probably, on completely flat terrain with no winds, maintain 20mph with something built the way your picture shows, maxing out the motor power all the time. If there's more drag than my SB Cruiser trike has, either from air resistance on the canopy/body or rolling resistance on the tires, it'll take more power than that.
If you use a middrive system (diff or no diff) motor chained or belted to a wheel or wheels, the same power will still be needed to maintain the same speed, but you can gear it for higher torque, and use a physically smaller motor (as long as the motor can still handle the power), as long as the motor can still spin fast enough . If a gear-shiftable diff or transmission is used (even a two- or three-speed bicycle drivetrain of some type), you can set it up for a low gear for acceleration and a high gear for cruising speed.
A hubmotor will be less maintenance as long as it is in a well-built wheel (the cheap kits probably won't be because they use too thick spokes for the rims they use), but you can get ones that will be from places like ebikes.ca just costs more, or you can build it yourself or have one built at a local bike shop using just a bare hubmotor and the rim you need to support the weight of your system that can survive the road conditions you'll have.
Whatever you use, you'll also need a battery that can continously output the power needed, probably at least 1000w or more, for the entire distance/time you need to ride it.
At a guess it will take 60-100Wh/mile at 20MPH to operate something built like yours, on flat roads with no wind and good road conditions. Rounding up, that means it will take at least 100Wh for every mile you want to go. If you want 30 miles of range, you need 3000Wh of battery. If it's a 52v system, that means 3000Wh / 52v = 58Ah of battery. Call it 60Ah. Etc.
You may need less power, and you may need less range capability, but you'll have to either use the simulators to guesstimate your usage/etc or experiment once you have it built, to see what actual usage and power requirements are.
ebikes.ca
