The precision of a quantum charge pump is studied through an exact solution of a time-dependent Schrödinger equation, and the noninteracting Anderson impurity model. An approximate solution to the interacting Anderson model in the strong Coulomb blockade limit is also presented. Near the adiabatic regime, the precision is rigorously found to depend exponentially on the operating frequency of the charge pump. The semiclassical rate equation is rederived when the temperature is higher than the energy quantum of the pumping frequency. Nonadiabatic heating to the leads is also discussed.