The paper deals with the metrological characterization of a digital wattmeter developed by using a personal computer, a shunt, two commercial data acquisition boards and commercial software. The uncertainty of the active power measurements is assessed individually characterizing the voltage channel, the current channel and the phase displacement measurement between the two channels. In particular, the uncertainty on the voltage and current rms measurements is evaluated by using an already proposed approach based on the Monte Carlo method and on the usage of five parameters, while the uncertainty on the phase angle is experimentally estimated. In order to validate the proposed approach for the uncertainty evaluation, various experiments were carried out and a comparison with the Italian National Standard wattmeter was made. The results show that the proposed PC-based solution can be suitable for the development of standard instrumentation, since its accuracy is comparable to the accuracy of much more expensive and sophisticated instrumentation