A question about points on an elliptic curve with prime denominator

Abstract

Let E be an elliptic curve defined by a Weierstrass equation with integer coefficients. Any rational point on E other than the identity is of the form $ ( x(P) / z(P)^2 , y(P) / z(P)^3 ) $ where $ x(P), y(P) \in \mathbb Z $ and $ z(P) \in \mathbb N $ and in addition both $ \gcd( x(P) , z(P) ) = 1 $ and $ \gcd( y(P) , z(P) ) = 1 $ hold. The question is: how often is $ z(P) $ a prime number?