Among various power reduction methods, variable bit-width arithmetic units have been proposed in approximate computing literature. In this paper, we add a variable bit-width memory unit in a RISC-V processor. Integrating both computation and memory units with variable bit-width leads to a power reduction: from 7% to 29% for Sobel filter application and from 13% to 24% for an application that computes the position of a robotic arm (forwardk2j). We also propose a global energy model for a RISC-V processor with variable bit-width units (for computation and memory). This model allows us to evaluate the impact of various parameters in both the software application (e.g., the amount of instructions that can be executed with a reduced bit-width) and the hardware architecture (e.g., impact of potential reduction for each unit).