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Disruption-tolerant networks (DTNs) rely on intermittent contacts between mobile nodes to deliver packets using a store-carry-and-forward paradigm. We earlier proposed the use of throwbox nodes, which are stationary, battery-powered nodes with storage and processing, to enhance the capacity of DTNs. However, the use of throwboxes without efficient power management is minimally effective. If the nodes are too liberal with their energy consumption, they will fail prematurely. However, if they are too conservative, they may miss important transfer opportunities, hence increasing lifetime without improving performance. In this paper, we present a hardware and software architecture for energy-efficient throwboxes in DTNs. We propose a hardware platform that uses a multitiered, multiradio, scalable, solar-powered platform. The throwbox employs an approximate heuristic for solving the NP-hard problem of meeting an average power constraint while maximizing the number of bytes forwarded by the throwbox. We built and deployed prototype throwboxes in UMass DieselNet, a bus-based DTN testbed. Through extensive trace-driven simulations and prototype deployment, we show that a single throwbox with a 270-cm2 solar panel can run perpetually while improving packet delivery by 37% and reducing message delivery latency by at least 10% in the network.