A system for determining an energy requirement of a vehicle for a journey. The system may comprise a predictor mechanism to predict, using an energy prediction algorithm, a vehicle energy requirement for the journey. The system comprises an updater mechanism configured to refine the energy prediction algorithm for the vehicle by determining for each of a number of historical journeys undertaken by the vehicle, an error between an actual vehicle energy usage for the historical journey and a predicted energy usage derived using the energy prediction algorithm for the historical journey. An aggregate error is calculated from the errors of the number of historical journeys. The updater is arranged to adjust the energy prediction algorithm to reduce the aggregate error.