In order to reduce both electricity bills and carbon emission, data center operators begin to build their own on-site green energy plants. However, challenges arise with the fluctuating workload, temporally diverse electricity price and intermittent green energy. To deal with these challenges, this paper presents an online workload scheduling algorithm which can minimize the total electricity bills of a data center without any future information about workload, electricity price or green energy availability. First, a model for the total electricity bills of a data center is introduced. Then a stochastic optimization problem to minimize the electricity bills is formulated. Finally, solution to the optimation problem is made to form the corresponding workload scheduling policy. Experimental results based on real-world traces show that the proposed algorithm can effectively reduce the total electricity bills while guaranteeing the workload performance.