When a power transformer is loaded so that it is operating at maximum efficiency the copper losses are equal to the iron losses The ‘efficiency’ is, of course, the ‘power efficiency’, i.e. the ratio of output power to input power. The same condition applies for maximum ‘energy efficiency’ only if the load is constant for the period being considered.With most transformers the load varies considerably over a load cycle and the optimum ratio of copper losses to iron losses calculated on the assumption of constant load for a given period can only be, at least, a rough approximation.The paper suggests a method of obtaining a more accurate optimum ratio in cases where the load variation of the transformer can be estimated in advance. The assumption is made that the copper resistance is constant.