I was wondering how much I would have to charge a basket ball of mass M, so that I would begin to feel the inertia of the electrostatic field energy. We can calculate the total electrostatic field energy surrounding the charged basket ball, call it E. Can we then equate this energy E with a mass by E = mc^2? So that when the electrostatic energy surrounding the charged basket ball divided by the speed of light squared is a significant fraction of the mass of the basket ball we would notice the basket ball being harder to accelerate then an uncharged basket ball of mass M? Edit, at the same time that we begin to notice the inertia of the electrostatic field would we also notice the charged ball was harder to spin? Thanks!