In-home displays (IHDs) have the potential to change the way energy use feedback is communicated to householders and to induce behavioural change by providing real-time information on energy consumption. How feedback is presented to users and how users understand the meaning of such feedback depends on the design of the display interface. This paper presents a laboratory study to investigate how the user interface of energy displays might be best designed. The work studied people's ability to spot changes in smart meters by comparing three different types of display design to see which was most effective in attracting attention to changing information, and whether the use of colour would also facilitate detecting changes. A computerised spot-the-difference task was undertaken; accuracy rates and response times were the key dependent variables, and qualitative information on participant preferences was gathered in interviews after the computerised task. Results showed that there was a difference between design types presenting the same information: simple numerical displays were superior to pictographic or analogue scale designs in both response time and accuracy. They also showed that the use of colour did not significantly affect performance. In general, participants also subjectively preferred numerical design for their in-home energy displays.