Pyroelectric nanogenerators have emerged as an effective approach for the energy of the new-era, the era of internet of things, with a great potential for harvesting waste heat energy. Here, we present the figure-of-merit for quantifying the output of a pyroelectric nanogenertor. The charge-voltage cycle during temperature cycling has been experimentally performed on a PVDF film and PZT ceramic material. Output energy of 2.53 mu J per cycle and 2.59 mJ per cycle is presented at temperature cycling of 23 K (from 300 to 323 K) and 8 K (from 300 to 308 K) for the PVDF and PZT, respectively. The practicability of the derived figure-of-merit as a universal standard for pyroelectric nanogenerator is further experimentally confirmed and compared with previous results. We believe that it could be useful for optimizing the performance of pyroelectric nanogenerators from both material and structural aspects as well as estimating the generated electricity based on maximum temperature cycling.