FEDS draws upon a number of sources to determine inferable parameter values. Major sources include national building energy consumption surveys such as the Commercial Buildings Energy Consumption, Residential Energy Consumption Survey, large end-use studies such as the End-Use Load and Consumer Assessment Program, ASHRAE handbooks, building and equipment codes and standards, and manufacturers' data and extensive building audit and evaluation experience.

A locked value, in terms of FEDS inputs, is one that the user has entered for an inferable parameter. This indicates to the model that this is a user-entered value and should not be updated (inferred). Clicking on the lock symbol can also lock a currently inferred value. When a value is locked, the lock icon will appear as a latched or closed lock. To unlock a value, simply click the icon again, changing it to an open or unlatched lock. This value will now be inferred the next time inferences are run.