Energy consumption in ICT is growing annually by 4% despite efficiency gains in technology, and it already has a carbon impact that is equivalent to air travel. It is therefore important to study ways to reduce energy consumption in ICT while preserving acceptable levels of quality of service (QoS). Energy consumption in computer systems is often related to the operating load and will be increasing as systems become more energy efficient. We examine the choice of system load that offers the best trade-off between energy consumption and QoS, and use measurements to validate the results. © 2012 IFIP.