We present an optomechanical accelerometer with high dynamic range, high bandwidth and readout noise levels below 7.8 x 10(-5) m s(-2)/root Hz. The straightforward assembly and low cost of our device makes it a prime candidate for on-site reference calibrations and autonomous navigation. We present experimental data taken with a vacuum sealed, portable prototype and deduce the achieved bias stability and the accuracy of the sensitivity. Additionally, we present a comprehensive model of the device physics that we use to analyze the fundamental noise sources and accuracy limitations of such devices.