Wireless sensor networks are fundamentally limited by their energy storage resources and the power they obtain from their environment. Several micro-solar powered designs have been developed to address this important problem but little analysis is available on key design trade-offs. We develop a taxonomy of the micro-solar design space identifying key components, design choices, interactions, challenges, and trade-offs. Based on this taxonomy, we provide an empirical and mathematical analysis of two prominent designs of micro-solar power systems (Heliomote and Trio), and interpret the results to propose design guidelines for micro-solar power systems.