The huge energy consumption in data centers produces not only high electricity bill but also tremendous carbon footprints. Although today’s servers are more energy efficient and data centers of leading internet companies are highly energy efficient than ever before, the fluctuations in external workload and internal resource utilization urge the calling for energy proportional computing. Thus good knowledge of server energy proportionality can help better workload placement and more energy savings. In this paper, we investigate all the 477 valid published results of SPECpower_ssj benchmark from 2007 to 2016Q3 and reorganize them by hardware availability year for more accurate analysis on production servers. Through comprehensive analysis we find that: (1) The specious stagnation of energy proportionality in recent years is mainly caused by the adoption of processors of specific microarchitecture and is not the indicative trend of energy proportionality improvement. (2) Microarchitecture evolution has more influence on energy efficiency improvement than energy proportionality. (3) Today’s servers’ peak energy efficiencies are shifting from 100% utilization to 80% or 70% utilization and server energy proportionality improves with such shifting. We then conduct extensive experiments on 4 rack servers to investigate the energy efficiency variations under different hardware configurations, including memory per core installation and processor frequency scaling. Our experiments show that hardware configuration has significant impact on server’s energy efficiency. Our findings presented in this paper provide useful insights and guidance to system designers, as well as data center operators for energy proportionality aware workload placement and energy savings.