Because the aim of biofuel production is to provide a substitute for the use of fossil energy, it is important to check that the proposed system manages to create energy and does not use more energy than it produces. Publications use different metrics to evaluate the energy performances of the assessed systems. The net energy ratio (NER), defined as the ratio of produced energy/consumed energy, totals energy consumption as seen at the facility gate. This means that the consumption of 1 MJ of electricity will be accounted for as 1 MJ of invested energy. Other studies measure energy consumption in terms of cu­mulated energy ratio (CER); in that case the price of using 1 MJ of electricity will depend on how it has been produced and will measure the total quantity of primary energy used to create the MJ of electricity. Both approaches have their own interest; a NER will focus on the system technology, whereas a CER will also include the effect of the technological environment of the production system. None of the approaches considers the fraction of storable energy (which could have been directly used for transportation) mobilized by the process. Table 13.8 summarizes the environmental and energy assessment methods.