The predicted annual growth of energy consumption in ICT by 4% towards 2020, despite improvements and efficiency gains in technology, is challenging our ability to claim that ICT is providing overall gains in energy efficiency and Carbon Imprint as computers and networks are increasingly used in all sectors of activity. Thus we must find means to limit this increase and preserve quality of service (QoS) in computer systems and networks. Since the energy consumed in ICT is related to system load, ]this paper discusses the choice of system load that offers the best trade-off between energy consumption and QoS. We use both simple queueing models and measurements to develop and illustrate the results. A discussion is also provided regarding future research directions.