Dielectric ceramics for capacitors have attained significant attention in current time owing to their large power densities and fast charge/discharge rates. Lead-free [(Bi0.50Na0.40K0.10)(0.94)Ba-0.06](1-x)LaxTi0.975Ta0.025O3 (abbreviated as BNKBTT-100xLa) relaxor ceramics are prepared using mixed-oxide technique. All ceramics are revealed a pseudo-cubic structure using X-ray diffraction technique. The long-range order existing in BNKBTT-100xLa ceramics is found to reduce with increasing composition. As a result, a highly effective energy storage density (W-rec) similar to 1.558 J/cm(3) and efficiency (eta) similar to 88.46% is achieved in BNKBTT-2La ceramic. In addition, BNKBTT-2La composition is fatigue-free up to 105 cycles. Also, BNKBTT-4La ceramic exhibits a dielectric constant (epsilon') similar to 2281 near 150 degrees C with Delta epsilon'/epsilon'(150 degrees C) below 15% over a large range from 39 to 383 degrees C, showing an outstanding dielectric temperature stability. Therefore, BNKBTT-2La and BNKBTT-4La ceramics are outstanding for energy density and thermally stable dielectric permittivity, respectively, for the device applications.