An analytical model supported by the numerical simulations describing the effectiveness of the terahertz radiation generation on the surfaces of metals is presented. Properties of the energy deposition and transfer are shown to play a primary role in quantitative evaluation of the THz radiation generation efficiency. Transitions between different nonlinear scaling laws for high and low energies and different sample thicknesses are considered. Using low-frequency energy generated by the optical rectification on metal surfaces allows us to investigate nonlinear effects in the quasistatic regime and reveal additional information on the transport and optical properties of metals.