This paper considers the idea that for many products there is a particular length of lifespan for which the environmental impact of the item is at a minimum. It is suggested that for some types of product this ideal lifespan is not, as is often believed,`as long as possible' but until the performance of a new product justi®es replacing the existing one. Electrical household appliances are identi®ed as a product group for which this issue is particularly relevant. It is proposed that the improvement over time of use-phase energy consumption shown by many products in this group means that at a certain point it is environmentally bene®cial for the customer to replace their existing machine with the latest model. The paper suggests that it is possible to calculate this point in the life of the product when replacement is most bene®cial from information about new and existing machines. The development of a method for calculating this optimum lifespan from energy consumption data is described, using a dishwasher to demonstrate the technique. Using the equation devised, the environmentally optimum lifespan of a dishwasher is found to be 8.1 years in 2003. Keywords: product lifespan, environmentally sensitive design, domestic appliances NOTATION A, B constants from the equation of the energy consumption graph b build energy (kW h) E energy consumption (kW h/year) m gradient of the energy consumption graph (kW h/year 2 ) n length of lifespan (years) t time from the beginning of the energy consumption graph (years) T start year (years) V variable energy (kW h/year)

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