Plug-in electric vehicles can provide the power grid with some degree of control authority over fluctuations in electric load, thanks to their charging flexibility. The magnitude of this control authority depends on a variety of factors including the number of vehicles plugged into the grid, their instantaneous power demands, and the degree of flexibility in these demands. This paper addresses the problem of using a universally broadcast control signal to directly control the charge rate of a fleet of plug-in electric vehicles connected to the grid. The paper specifically seeks a control algorithm that is robust to uncertainties in renewable energy generation and the number of grid-connected vehicles. We adopt the sliding mode control strategy to achieve stability and robustness with respect to the collective effects of system uncertainties. The control law and robustness conditions are derived using the Lyapunov stability criterion. The paper shows that using only the real-time imbalance between the electricity supply and demand as a measured system output, the controller is able to precisely attenuate this imbalance, achieving reliable demand-side load management. Numerical simulations are provided to evaluate the performance of this controller.