Current performance specifications of seat belt pretensioners include web retraction and belt load. These criteria may adequately represent the performance requirements of a pretensioner in a restraint system. However, by themselves, they are inadequate to evaluate potential design modifications to improve efficiency levels and thus increase energy output of the pretensioner.This paper demonstrates a non-linear phenomenon associated with web retraction and the belt load during pretensioning. It is this non-linear behavior that promotes the insensitivity for use of web retraction in predicting the energy output of a pretensioner with design modifications. This paper proposes an energy measuring method that can be used to more accurately measure the energy output of a pretensioner and then be used to evaluate the effects on output of a pretensioner due to design modifications.