From an environmental perspective, the durability of products is generally seen to be a positive and desirable goal, especially from the point of view of waste management. However, the extension of the lifetime of energy-using products is not necessarily the optimal strategy, as the efficiency of products generally decreases with wear and tear, and their substitution by more energy-efficient products can be more environmentally beneficial in the long run. There is currently no standardised approach to resolving this conflict, although various approaches have been illustrated in the literature based on different perspectives. The present article describes an original method for environmentally assessing the durability of energy-using products in order to identify if and to what extent the potential extension of the product’s lifetime could have life-cycle benefits. The method is based on the comparison, within a life-cycle perspective, of two scenarios of different lifetimes of a target product and its potential substitution with (one or more) better performing alternatives. The method considers some key parameters of durability, including the average lifetime of the product(s), the annual energy consumption, the impacts of lifetime extension (e.g. through repair) and the environmental performance of the replacement product. A general index and a simplified index have been derived from the method. The applicability and relevance of the simplified durability index is shown in two case studies (of washing machines). The results of the assessment can be used for ecodesign purposes by manufacturers (e.g. for the identification of technical design options for extending the lifetime of products) or by policy makers (e.g. in setting requirements for product policies). The applicability and robustness of the method are discussed, including potential limitations (e.g. allocation of co-products/co-services, variability of impacts on manufacturing or end-of-life), difficulties (e.g. for products with short technological cycles), and possible improvement (e.g. the impacts of repair on extending the lifetime, the effects of different usages). The two case study applications of the method show that some life-cycle environmental benefits can be gained by extending the lifetime of the products, even if it would delay their replacement with more energy-efficient products. However, the benefits and their relevance are variable, mostly depending on the selected impact category, the extension of the lifetime, the impact of repair, and the efficiency of the replacement product.