CL spectra from nano-granular and electronic grade bulk ZnO have been measured as a function of electron beam current, IB, and specimen temperature (80K-300K). A power-law model (ICL=k.IBm) was used to investigate the influence of particle size on the excitation power dependence of each of the observed ZnO CL emission bands. Bulk ZnO displayed a near band edge (NBE) emission peak at 3.28eV with no other CL emission. The intensity of the bulk ZnO NBE exhibited a superlinear dependence on IB with a power law exponent of m = 2.2 at 300K, while its peak position remains fixed with increasing IB. Conversely, the NBE in the nano-granular ZnO displayed a strong red shift with increasing IB at 300K. Its position, however, did not change with IB at low temperature. Power exponents of m = 0.9 and m = 1.4 were measured for the NBE in the nano-granular ZnO at 300K and 80K, respectively. CL emission from defect-related centres positioned at 1.8eV and 2.3eV was observed at 300K in the ZnO nano-particles but not in the bulk ZnO. The nano-granular ZnO also exhibited an intense featureless emission which steadily increased up to the IR detection limit at 0.75eV. Its intensity displayed a supralinear dependence on IB with m > 9. This emission was completely quenched by a slight decrease in specimen temperature, and was not observed in the bulk ZnO under all excitation conditions. The IR emission is attributed to black-body radiation where the low thermal conductance of the ZnO nano-particles leads to significant electron beam heating. The observed red shift of the NBE peak position in nano-granular ZnO is therefore attributed to its dependence on temperature rather than carrier density. Variation in the NBE power law exponents between bulk and nano-granular ZnO is explained by the higher defect density of the ZnO nano-particles as well as their large surface-to-volume ratio.