This paper is the first to evaluate the impact of a large-scale field deployment of mandatory time-of-use (TOU) pricing on the energy use of commercial and industrial firms. The regulation imposes higher user prices during hours when electricity is generally more expensive to produce, and is the most common way for time-varying incentives to be transmitted to retail electricity customers. We exploit a natural experiment that arises from the rules governing the program to present evidence that TOU pricing induced negligible change in overall usage, peak usage or peak load. As such, economic efficiency was not increased by this regulation. Bill levels and volatility exhibit only minor shifts, suggesting that concerns from advocacy groups about increased expenditure and customer risk exposure have been overstated.