The methodological framework of LCA has a number of well-known problems and limitations. This has generated a lot of scientific efforts to eliminate or alleviate these. This paper can be seen as one contribution to these efforts, with a particular focus on energy issues in LCA. It has been concluded that a future comprehensive methodological framework for LCA must include a number of principles for comparison and aggregation of different energy forms or of their environmental impacts. In economics, the central concept of value is encapsulated in the term numeraire. This signifies the function of money as a common, universal currency, a measure of value or a unit of account. A numeraire can be used both for comparison and aggregation. A basic requisite for a numeraire is generality. It is here discussed whether it is possible to define an energy numeraire for use in LCA methods, in particular when the LCA is to be used for specific kinds of optimisation. I conclude that the exergy concept needs quite a lot of further theoretical development before it can be included in LCA methodology. Attempts to do so at the present stage would be premature and lead to disappointment and confusion