Smart products are becoming more present in everyday life. They are prevalent in different markets such as electronic devices, cars and household appliances. One important dimension of product smartness is "multi-functionality". When choosing a product, the consumer takes into account subjective and objective purchase drivers. Price, brand, aesthetics, environmental impact and functionalities represent an important set of these. In this context the work are interested in correlating two drivers: functions and eco-sustainability. Generally, this last characteristic should be maximized taking into consideration a correct balance with product functionalities. The aim of this work is to investigate this correlation. It can be measured by a suitable correspondence in order to determine a quantitative law. This relationship can be useful to the designers to determine the product features during the product design phase, but it can be also used by consumers to compare similar products. This paper reports the problem domain, approach for correlation law definition and, finally, the experimental analysis of product functions vs. environmental sustainability. Two case studies in, the household appliances sector will exemplify the proposed analysis.