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Noncentrosymmetric quantum materials can convert AC input current into DC transverse current through the nonlinear Hall effect at zero magnetic field. We analyze the AC-DC power conversion efficiency of such ``Hall rectifier'' and suggest its application in wireless charging and energy harvesting. Our key observation is that the development of Hall voltage results in a change of longitudinal resistance, resulting in a violation of Ohm's law due to the nonlinear Hall effect. This feedback mechanism balances the input power and the output power and hence is crucial to understanding the power transfer from source to load. We derive a general expression for the power conversion efficiency in terms of material parameters, external load resistance, and input power. As the Hall current is perpendicular to the electric field and does not generate Joule heating by itself, we obtain high power conversion efficiency when the Hall angle (which increases with the input power) is large and the load resistance is optimized. Promising materials for high-efficiency Hall rectifiers are also discussed.