Cruising at 20 mph / 32 kph, with light pedaling, generally takes about 400 (300-500) watts. With wind, or just a very slight uphill, 600w is not unusual. I'd say aim higher, so you don't come to a complete stop in a 20 mph headwind. 36v with a 20 amps controller will get you 700w. I consider it the minimum for serious riding. That's too much for the very smallest motors, but many of the smaller motors take that wattage fine. You don't have to use a big heavy direct drive motor. But if you live where the hills are steep, you will want the larger gearmotors, or a dd. 36v 15 amps, is still tolerable for many, 540 watts will do the speed you want for sure, if the motor is not wound too slow RPM, or the controller does not have a speed limiter enabled.
However, you can easily ride that 700w bike using only 200 watts much of the time when conditions are good. But the whole idea of an ebike to me, is to not have to ride only when there is no wind, or no hill. Typically, 300w might be all you need, but when you need 700w, you NEED it.
