Coal is the biggest single source of energy for electricity production and its share is growing. The efficiency of converting coal into electricity matters: more efficient power plants use less fuel and emit less climate-damaging carbon dioxide. This report explores how efficiency is measured and reported at coal-fired power plants. With many different methods used to express efficiency performance, it is often difficult to compare plants, even before accounting for any fixed constraints such as coal quality and cooling-water temperature. Practical guidelines are presented that allow the efficiency and emissions of any plant to be reported on a common basis and compared against best practice. A global database of plant performance is proposed that would allow under-performing plants to be identified for improvement. Armed with this information, policy makers would be in a better position to monitor and, if necessary, regulate how coal is used for power generation. The tools and techniques described will be of value to anyone with an interest in the more sustainable use of coal.