Power(P)=Force(F) . Velocity(V)
"In the straightforward cases where a constant force moves an object at constant velocity, the power is just P = Fv. In a more general case where the velocity is not in the same direction as the force, then the scalar product of force and velocity must be used." Source-Hyperphysics
The question here is, since the velocity is constant, there is no acceleration, then where does the force come from?