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The Eco-design directive issued by the European Commission has led to requirements on efficiency of power transformers. In the case of large power transformers used in grid applications, serious problems are encountered in establishing how reliable the loss measurements are. An effort is currently ongoing within IEC to produce a documentary standard on "Rules for the determination of uncertainties in the measurement of the losses of power transformers". An IEC standard should be clear and easy to understand by all users in the industry. Background theory and material, whilst necessary for understanding, is not required for the day-today application of the standard. This paper presents a more detailed background and theory on the measurement of transformer losses and how to quantify precision. The authors are all members of the IEC maintenance team working with the standard.