An investigation of the impact of heat dissipated by a pump system on the overall performance of the oil well has been carried out using a dynamic, non-linear electric submersible pump (ESP)/well model based on a systems dynamics modelling package. This Package allowed rapid model development for complex well situations. It included building a detailed ESP model. Comparison of results from the detailed ESO model with commercial software gives good agreement. The dynamic ESP/well model also gives good numerical agreement during steady-state production as well as reproducing the unstable production behaviour observed in the field. A detailed study showed that the heat dissipated by a down-hole motor and pump had a significant impact on the overall production performance of the extra heavy and heavy oil wells when the magnitude of the change in viscosity of the crude oil with respect to change in temperature was sufficiently large. This was particularly true for viscous oils, but this effect could be important even for lighter North Sea crude oils under unusual circumstances, e.g. pressure boosting with an ESP in a subsea pipeline when the crude oil is cold.