We investigate the power consumption of a network server based on a multiclass open queuing network with closed-form solution. The model consists of service stations that represent major subsystems within the server. We also propose a practical methodology for fixing model parameters through a set of simple tests. The resulting model can reasonably predict the power usage trends with respect to workload intensity that can be observed in real computing systems. Supporting experiments provide a cross validation of the model against performance and power measurements obtained on three types of commercial systems. © 2012 Elsevier Inc. All rights reserved.