As cloud services multiply rapidly, so does the computing centers dedicated to them, and consequently their power consumption. Although this consumption is hampering data centers' expansion, these infrastructures have not yet reached energy proportionality, thus wasting significant amounts of energy. Numerous energy metrics have been propose as incentives towards greener infrastructures, but none of them currently gives direct insights about the energy proportionality and green energy usage of data centers. In this paper, we propose GLENDA: a Green Label towards Energy proportioNality for IaaS DAta centers. We validate our metric by using traces from real infrastructures, and show that our label gets a better grade when increasing energy efficiency, increasing utilization rates, and using distributed renewable generation. We expect this new metric to become a useful reference for Cloud providers towards green data centers.