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In this paper, we address the problem of providing video-on-demand (VoD) services to numerous clients energy-efficiently. To reduce energy consumption, multiple requests for the same video are batched and served by one single multicast stream. However, this brings additional delay to most clients. Utilizing client pre-caching is an efficient way to eliminate the delay: while the server is batching multiple requests, the clients can play the locally cached prefix of the requested video. The multicast session containing the later part of a video can be delayed till the prefix is played out. Our evaluation demonstrates that with a carefully designed pre-caching scheme, even a small cache (with the size of a video) can reduce 50% energy consumption. Moreover, we determine the optimal client cache allocation scheme to maximize the utilization of client cache and further minimize the energy consumption.