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User cooperation improves performance in wireless systems, but it requires other users to expend energy acting as relays. When energy is scarce, users have incentive to refuse to cooperate in order to conserve resources. Therefore, we investigate cooperative communications from an energy- efficiency perspective. We present a cellular framework in which two mobile users, who desire to communicate with a common base station, may cooperate via decode-and-forward relaying. We maximize users' bits-per-energy efficiency by defining the achievable bits-per-energy region and finding the power allocations that achieve its Pareto boundary. To find an efficient approach that gives selfish users incentive to cooperate, we apply game theory, finding the power allocations that achieve the Nash bargaining solution. Numerical results indicate that the Nash bargain provides a fair and efficient compromise and that both users obtain noticeably improved bits-per-energy efficiency via cooperation.