Relaxor ferroelectrics have drawn attention for possible applications in solid-state cooling and thermal energy harvesting, owing to their electrothermal energy conversion properties. Here, we have synthesized and characterized the structure-property correlations of a new Sn- and Nb-doped (Ba,Ca)TiO3 relaxor ferroelectric with large pyroelectric and electrocaloric effects over a broad temperature range. We observed two peaks for the temperature-dependent pyroelectric coefficient: (i) -(∂P/∂T) ∼ 563 μC/(m² K) at T∼ 270 K and (ii) -(∂P/∂T) ∼ 1021 μC/(m² K) at T∼ 320 K. In addition, a broad peak for electrocaloric temperature change is observed near 320 K with a relative cooling power of ∼17 J/kg. These properties could be correlated to structural changes observed using X-ray diffraction at two different temperature ranges in the material. Analysis of high-energy X-ray scattering and specific heat capacity data revealed a transition from the cubic to tetragonal phase near Tm∼ 320 K, whereas an additional increase in the tetragonality (c/a) of the polar phase is observed below Ts∼ 270 K.