The present study is a new experimental investigation into energy dissipation due to wave breaking. Long-crested wave groups based on JONSWAP spectra are generated in a wave flume to estimate the energy loss from the wave packet due to the occurrence of single breaking events. These events range from gentle spilling to large plunging breakers. The fractional energy losses and breaking strength parameter are shown to be in good agreement with previous studies based on other spectral shapes. Due to the significance of wave breaking in the evolution of large ocean waves, the dissipative model of Tian et al. (2010) [1] is examined and improved upon for JONSWAP spectra. Local wave geometry and breaking duration obtained from measurements using an accurate two-camera setup are employed to obtain new parameterisations, directly applicable to realistic ocean waves. An increase of approximately 85% in the proportionality constant needed to compute νeddy is deemed necessary such that energy losses from the new experimental wave groups are accurately captured. Furthermore, it is shown that the time and length scales of the breaking process are well correlated to the local steepness of the wave event, and new fits are obtained that considerably reduce the scatter of the original model. As focused wave groups represent the most probable shape of large wave events in the open seas, the improved model can be implemented in computationally-efficient numerical potential flow solvers to simulate energy dissipation from breaking events in realistic waves.