Power usage within a Data Center (DC) goes beyond the direct power needs of servers to include networking, cooling, lighting and facilities management. Data centers range from closet-sized operations, drawing a few kilowatts (kW), to mega-sized facilities, consuming tens of megawatts (MWs). In almost all cases, independent of size there exists significant potential to improve both the economic and environmental bottom line of data centers by improve their energy efficiency, however a number of challenges exist. This paper describes the resulting maturity model, which offers a comprehensive value-based method for organizing, evaluating, planning, and improving the energy efficiency of mature data centers. The development process for the maturity model is discussed, detailing the role of design science in its definition.