In this study we analyse twenty years of half-hourly demand data for the eastern states of Australia, and relate the electricity demand to temperature.  After time of day, temperature is the next most important driver of energy demand, with hot days resulting in the highest demand for electricity.  We use different measures of temperature to find the best way to explain the variation in observed energy demand: actual temperature, apparent temperature (combination of temperature and humidity) and effective temperature (concurrent temperature combined with effect of previous days overnight minimums).  The approach we use calculates a new set of coefficients  for each year of data.  From this approach we can show that the relationship between temperature and energy demand has evolved with time, with demand becoming more and more sensitive to temperature.  This is likely due to a continuing increase in the penetration of air-conditioning, the lack of appropriate building standards and an increasing wealthy society meaning the electricity is a smaller part of average family budget so that comfort through air-conditioning is relatively cheap and therefore more widely used.