Laser welding is a common technique for joining metals in many manufacturing industries. Due to the heat input and the resulting melting and solidification, the parts deform causing residual distortion and residual stresses. To assure the geometrical and functional quality of the product, Computational Welding Mechanics (CWM) is often employed in the design phase to predict the outcome of different design proposals. Furthermore, CWM can be used to design the welding process with the objective of assuring the quality of the weld. However, welding is a complex multi-physical process and in a design process it is typically not feasible, for example, to employ fluid simulation of the weld pool in order to predict deformation, especially if a set of design proposals is under investigation. Instead, what is used is a heat source that emulates the heat input from the melt pool. However, standard heat sources are typically not flexible enough to capture the fusion zone for deep keyhole mode laser welding. In this paper, a new heat source model for keyhole mode laser welding is presented. In an industrial case study, a number of bead on plate welds have been employed to compare standard weld heat sources and develop the new heat source model. The proposed heat source is based on a combination of standard heat sources. From the study, it was concluded that the standard heat sources could not predict the observed melted zone for certain industrial application while the new heat source was able to do so.