This article outlines several enhancements made to autonomous renewable energy systems (ARES)-I, the Cardiff School of Engineering's hybrid photovoltaic and wind energy simulation program. The resulting program, ARES-II, unlike the majority of other hybrid simulation programs, predicts the battery state of voltage (SoV) rather than its state of charge (SoC). Loss of load (Lot) occurs when the battery voltage drops below the low voltage cut off limit. Given load and weather profiles, ARES-II is able to predict the occurrence of loss of load thus giving a direct measure of the system autonomy. The enhanced model also predicts the effect of different battery temperatures on the Lot. Experimental work has shown a significant change in storage battery resistance and capacity with temperature. Incorporating battery temperature effects into the battery algorithm is a novel advancement in hybrid system voltage simulation. Further refinements have also been made to the voltage controller algorithms. Accurate modelling of the non-linear action of the low voltage controller is of paramount importance when predicting loss of load probability for hybrid systems. Combining all the above features and incorporating them into ARES-II produces a simple, accurate and reliable method for hybrid system design and Lot prediction as a function of the combined variability in the weather and load.