This paper considers the problem of extending the battery lifetime for a portable computer by offloading its computation to a server. Depending on the inputs, computation time for different instances of a program can vary significantly and they are often difficult to predict. Different from previous studies on computation offloading, our approach does not require estimating the computation time before the execution. We execute the program initially on the portable client with a timeout. If the computation is not completed after the timeout, it is offloaded to the server. We first set the timeout to be the minimum computation time that can benefit from offloading. This method is proved to be 2-competitive. We further consider collecting online statistics of the computation time and and the statistically optimal timeout. Finally, we provide guidelines to construct programs with computation offloading. Experiments show that our methods can save up to 17% more energy than existing approaches.