IoT devices will increasingly be placed in remote or inaccessible locations for long-term deployments. At the same time, power and energy-reserve demands will be heterogeneous and scale proportionately with capability and functionality. Moreover, demand for sophisticated behavior will increase, placing stringent requirements on their energy subsystems. Thus there is a need for an energy subsystem that can be integrated with IoT nodes to provide energy-neutral operation. We describe an architecture, design, and implementation of an energy-neutral battery-backed solar energy system for IoT devices. An emphasis is placed on capacity planning, enabling real-time analytics and control, and packaging for thermal management in extreme environments. We present test results on a prototype implementation that is now in production for use with sensor-actuator nodes in environmental and ecological monitoring and control applications.