Though there is a lot of research related with lead-free piezoelectric materials, high power characteristics of the candidate lead-free materials have not been looked into in the literature under equilibrium conditions. This paper reports high power characteristics of a sodium potassium niobate (NKN) based ceramic under equilibrium conditions, and its application as a candidate material for piezoelectric transformers compared with hard lead-zirconate-titanate (PZT). (Na0.5K0.5)(Nb0.97Sb0.03)O3 was prepared with 1.5 mol % CuO addition. Disk-shaped samples were sintered with conventional sintering methods. High power characteristics were investigated with our high power piezoelectric characterization system (HiPoCS). Distinctly different from PZTs, the NKN ceramics did not exhibit a decrease in mechanical quality factor (Qm) with increasing vibration velocity (up to 0.4 m/s). Ring-dot piezoelectric transformers made from a disk shaped NKN ceramic revealed power density as high as 25 W/cm3, almost comparable to that obtained for the conventional PZT transformers. In conclusion, NKN ceramics possess good high power characteristics, which are satisfactorily applicable to piezoelectric transformers.