Smart grid is widely considered to be the next gen- eration of power grid, which will be integrated with information feedback communication to improve agility, reliability, efficiency and security. One of the key foundations of smart grid is timely access to meter data via reliable communication infrastructure. In this paper, we introduce cognitive radio into home area networks of smart grid, whose framework and result can be trivially extended to neighborhood and wide area networks. By means of spectrum sensing and channel switching techniques, smart meters can decide to transmit meter data either on the original unlicensed channel or the additional licensed channel, in order to improve reliability and timeliness. We find optimal sensing time to reduce packet loss rate and delay, under the constraint that the PU is sufficiently protected. We formulate the sensing-delay tradeoff problem and prove that it has unique optimal sensing time which yields the minimum delay. Simulation results are provided, which match very well with the theoretical results.