Accurate modeling of process temperatures within jacketed batch reactors has the potential to mitigate the risk of thermal runaways and enhance process control. A non-adiabatic heat-transfer model is derived for the investigation of heat transfer in laboratory to pilot-scale reactors of 0.5-40 L. By accounting for heat removed from the process by a total condenser and losses through the process lid, the model is able to predict process temperature profiles within the uncertainty limits of the experimental measurements. Heat losses from the outer jacket wall had a negligible impact on the evolution of process temperature but may contribute significantly to utility costs. Jacket duty measurements implied greater heat accumulation within the reactor vessel than anticipated, equivalent to similar to 60% of that in the process fluid at 40 L scale. This raises the potential for heat-transfer coefficients to be systematically under-estimated by adiabatic models, particularly at the laboratory to pilot scale.