This paper reports on the design, deployment and evaluation of "Wattsup", an innovative application which displays live autonomously logged data from the Wattson energy monitor, allowing users to compare domestic energy consumption on Facebook. Discussions and sketches from a workshop with Facebook users were used to develop a final design implemented using the Facebook API. Wattson energy monitors and the Wattsup app were deployed and trialled in eight homes over an eighteen day period in two conditions. In the first condition participants could only access their personal energy data, whilst in the second they could access each others' data to make comparisons. A significant reduction in energy was observed in the socially enabled condition. Comments on discussion boards and semi-structured interviews with the participants indicated that the element of competition helped motivate energy savings. The paper argues that socially-mediated banter and competition made for a more enjoyable user experience.