We present a classical dipole model, in combination with first-principles calculations, to study the local polarization, as well as the polarization potential, in InGaN alloys. Our results are in excellent agreement with previous ab initio calculations in the macroscopic limit and, in addition, offer an insight into the microscopic origin of the previous results, in particular the strong relation between internal strain and the upward bowing in the spontaneous polarization of InGaN. This model can be generalized to any piezoelectric material, and in particular to AlInN and AlGaN alloys.