The subject matter of this specification can be embodied in, among other things, a method that includes supplying power to a portion of a data center through a power distribution line. Utilization of a statistically significant sample of the computers is monitored, and an estimated individual power draw for each of the computers based on the utilization is calculated. An estimated total power draw is calculated for different times from the estimated individual power draws to generate a collection of estimated total power draw values for the different times. Actual power draw is monitored at the power distribution line and a collection of actual power draw values is generated. A function is fitted to pairs of actual power draw values and estimated power draw values, each pair comprising an actual draw value and an estimated draw value for the same time, and the function is then stored.