Harvesting energy from the environment makes it possible to deploy tiny sensors for long periods of time, with little or no required maintenance; however, this free energy makes testing and experimentation difficult. Environmental energy sources vary widely and are often difficult both to predict and to reproduce in the lab during testing. These variations are also behavior dependent-a factor that leaves application engineers unable to make even simple comparisons between algorithms or hardware configurations, using traditional testing approaches. This demonstration presents Ekho, a device that makes it possible to conduct realistic and repeatable experiments involving energy harvesting devices. Ekho is a generalpurpose tool that supports a wide range of harvesting technologies. We demonstrate, using a working prototype, that Ekho is capable of reproducing many types of energy harvesting environments accurately and consistently.