In the design of low power systems, it is important to analyze and optimize both the hardware and the software component of the system. To evaluate the software component of the system, a good instruction-level energy model is essential. In this paper we present a methodology for instruction level modelling of microcontrollers using gate level power estimation tools. We use the microcontroller, M68HC11, to illustrate this method. We study two different implementations of the microcontroller and show that the energy consumption of each instruction is quite different. Our study reveals that data correlation does not significantly affect the energy consumption of most instructions. Finally, we show the correctness of this model by running some sample programs and showing that the predicted energy estimates are quite close to the actual estimates.