The energy saving assessment for each chosen transformer is a very important issue in the rule-making process, not only because of the obvious environmental consequences but also considering the economic impact of the standards. The life cycle cost equation, which is a keystone feature, is also affected by the correct definition of the savings achieved by improving the efficiency of each class of transformers. Setting up efficiency standards by looking at what is offered in the current distribution transformers market doesn't necessarily entail an improvement of the system efficiency, i.e. fewer losses over the expected transformer lifetime. For this reason, the relationships between various parameters (losses, their ratio and the per unit values) and how their interaction can affect energy efficiency will be discussed.