Power consumption is an increasingly limiting factor in mod- ern ICT infrastructure, especially in the context of High Performance Computing. Common strategies to curb energy consumption are power capping, i.e. constraining the system power consumption within certain power budget, and Dynamic Voltage/Frequency Scaling, i.e. reducing the computing elements operating clock to decrease power usage. In this paper we tackle the frequency assignment problem in the context of a power capped system. We propose three approaches to solve the prob- lem, a greedy algorithm, a CP model and MIP model. As a case study, we consider the Eurora supercomputer, hosted at CINECA computing center in Bologna. The experimental results show that the MIP approach outperforms the other methods when the problem is loosely constrained. With tighter bounds the CP method can always find a solution, whereas the MIP fails to provide a solution for half of the considered instances.