Outside of the regulatory, sufficient power for an ebike depends on multiple factors: fitness of the rider, terrain, road safety, cargo load, and sometimes, damnit, I just want to get to and from work.
Personally, I like a 750w motor that can run 1200+ watts. I recently put a rear Grin All-Axle hubmotor and Phaserunner controller on a 52v bike. It's quite the fast ebike, with a bi-directional throttle for accelerating and braking. The work I do is physical, so it's now my work bike.
Otherwise, I have lately been riding this approaching eighteen year-old ebike:
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24 volt, 250 watts of slow-speed throttleless front hubmotor fun!
It all depends on what you want or need to do. So I'm keeping a close eye on legislation and/or enforcement.
And this sweet thing might soon get a legal maximum speed limit set via controller:
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