The environmental efficiency of a prototype electrochromic window was studied using eco-efficiency methodology, combined with life cycle assessment. The data obtained on the specified eco-efficiency indicators provide significant information that could be used in decision-making for the optimisation of the window's energy and environmental performance. The energy efficiency of the product is affected by its life expectancy and the climatic zone. It was found that in cooling-dominated areas the energy needs of buildings can be reduced by more than 55%, while the total energy saved can be 30 times the energy consumed during an expected 25 years life cycle. The corresponding CO
and human toxic emissions reductions were estimated to be 6 times those achieved with a conventional double-glazed unit. An expected retail price of 200 euros per m
for an electrochromic window would result in a cost of less than 0.10 euros for each kWh saved over a 20-year lifetime. Consequently, purchase cost reduction will be necessary if such devices are to meet market expectations for solar control window products.