A method for measuring characteristics of a power line signal having a fundamental frequency is provided. The method includes estimating the fundamental frequency of the power line signal, and generating a sampling clock that is substantially synchronized to the estimated fundamental power line frequency. The power line signal is sampled with the substantially synchronized sampling clock to generate data samples that include one or more substantially complete cycles of the power line signal. The data samples are processed to detect zero crossings of the power line signal, and the detected zero crossings are used to calculate an r.m.s. voltage of the power line signal measured over one full cycle, commencing at a zero crossing, and refreshed each half-cycle. Other aspects are also provided.