The functional unit is the key element allowing the practitioner to perform the LCA calculations. In comparative LCA studies, the choice of the right functional unit is crucial and can lead to intensive discussions. The following example in the packaging sector illustrates this issue: an LCA study on “beverage can” product system cannot use one kilogram of packaging material as functional unit but has to use a more appropriate unit as grams needed per litre of drink. In this sector, it is questionable if a 33cl beverage can is comparable to a 1.5 litre bottle taking into account the different purposes and properties of the 2 product systems. Not only the packaging material needs to be taken into account but also the quantitative and qualitative functionalities of the packaging, e.g. preservative properties vs. type of beverage, volume capacity, transportability, design, etc.
ISO standards require an assessment of the comparability of the studied product systems before making public any comparative assertion coming from an LCA study.
