With the help of this calculator you can get a rough estimate of the maximum achievable speed for a given power or the necessary power to achieve a desired speed. An upright posture on an uncovered bicycle is assumed.
The rolling resistance is independent of the speed. However, air resistance increases quadratically and plays a decisive role alongside the total weight in the plane. That is why high speeds of e.g. 50 km/h on flat roads can only be achieved with several hundred watts. However, a pedelec delivers a permanent maximum of 250 W. Everything above this has to be done by the cyclist by muscle power alone. Realistic power values over a period of one hour for average persons are below 200 W. The red line in the diagram indicates the maximum continuous power for pedelecs.