State-of-the-art studies have promoted the output energy density of triboelectric nanogenerators (TENGs) to 105 J m−3, while the current major barrier lies in the breakdown discharge limit. Universal performance metrics that reveal the maximized energy output ability of TENG approaching the breakdown limit are highly desirable but remain a major challenge. Herein, this work proposed performance metrics for charge density and output energy density, quantitatively characterizing the output performance based on a sliding-freestanding TENG with the charge excitation strategy. A series of parameters of different dielectric materials was systematically investigated to reveal their impacts on the performance metrics. Consequently, the maximum output energy density of 10 kinds of tribo-dielectrics was evaluated based on the voltage–charge (V–Q) curve, validating the proposed performance metrics. Guided by this new standard, we developed a stretched poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) film with synergistically improved permittivity and breakdown strength, achieving a record-high charge density and output energy density of 2.8 mC m−2 and 6.2 × 105 J m−3, respectively. Furthermore, a self-driven charge excitation system was explored in rotary-mode TENGs, showing excellent output capability to directly light up 15 series bulbs. This work establishes a basic standard and guideline for improving the energy output of TENGs, highlighting their potential applications for energy harvesting.