Using energy more efficiently is a key strategy for reducing global carbon dioxide emissions. Due to limitations on time and resources, actions must be focused on the efficiency measures which will deliver the largest gains. Current surveys of energy efficiency measures assess only known technology options developed in response to current economic and technical drivers. However, this ignores opportunities to deliver long-term efficiency gains from yet to be discovered options. In response, this paper aims to calculate the absolute potential for reducing energy demand by improving efficiency, by finding the efficiency limits for individual conversion devices and overlaying these onto the global network of energy flow. The potential efficiency gains for each conversion device are found by contrasting current energy demand with theoretical minimum energy requirements. Further insight is gained by categorising conversion losses according to the underlying loss mechanisms. The result estimates the overall efficiency of global energy conversion to be only 11 per cent; global demand for energy could be reduced by almost 90 per cent if all energy conversion devices were operated at their theoretical maximum efficiency.