Powering networked equipment after business hours is a primary source of electricity wastage in office buildings. In situ surveys, deployment of software agents and power measurements are generally conducted to estimate how well an IT infrastructure performs before enforcing energy-efficient solutions. Those approaches have cost and deployment constraints, obstacles for companies unwilling to invest time and money if return on investment is uncertain. This work proposes a novel approach based on virtual local area network (VLAN) auditing, capable of producing at low-cost and non-invasively a preliminary assessment of after hours networked equipment electricity wastage, providing factual insights to support the deployment of more expensive solutions. Experimentation within a University department, comprising more than 450 unique networked machines, has shown that (1) 10% of client machines regularly used have been constantly powered on, 68% have had nighttime activity and 53% week-end daytime activity; (2) 38% of machines have exhibited clear patterns of electricity wastage at nighttime, representing 5-6% of the School total power consumption, and (3) 35% of machines have exhibited clear patterns of electricity wastage during week-end days, representing 12-15% of the School total power consumption. (C) 2011 Elsevier Ltd. All rights reserved.