How do we assess energy efficiency? The methodology proposed in this book links the efficiency at the system level to the data - flows and established knowledge - found at the process level. This analysis determines the dependence of the system efficiency on physical characteristics of its processes. Unless this is done, these characteristics may be sources of large errors, by factors of one hundred or more. The suggested methodology saves time of analysis and gives a realistic assessment of the remaining uncertainties. Complete energy systems cannot dissipate more energy than they extract, directly or indirectly. Historic exploitation of underground coal could not run a steam engine for operations which require more coal than it can lift. Can the agro-ethanol industry operate without external energies, (ie: is it more than self-reliant)?