"Solar circuits" is a nomenclature for a photovoltaic panel, power manager, and microprocessor integrated on a System on a Chip that meet a predefined thermal design power envelope and volumetric design envelope for energy and space-constrained mobile devices such as cell phones, tablets, and laptops. The emerging market of Internet of Things, for over two decades, has prioritized interfaceless devices to either operate within extreme energy budgets or to rely on remote clients/terminals for offloading compute-intensive tasks. An emphasis on designing with energy generation and integration with interfaces in solar circuits expands the accessibility of IoT and mobile computers to developing markets as well as solutions requiring a "local first" design ethos. Solar circuit design will be contextualized within the precedent of Software Defined Hardware to specify an autonomous or batteryless capability requirement that utilizes energy harvesting or energy scavenging to meet "just enough" application needs. Furthermore, setting limits on system power consumption allows a solar circuit to serve as a proxy or "litmus test" for measuring (and subsequently demonstrating) the progress of Moore's Law within solar constraints. Performance of general purpose microprocessors that can be solar powered in mobile form factors is estimated to be consistently 25-30 years behind (in MIPS/W/mm^2) leading edge chips such as AMD Instinct and Apple A17. A number of heuristic methods are described in this paper to estimate the solar circuit requirements (in transistors, memory, and OPS) to run general purpose operating systems such as Microsoft Windows 95 and Linux 5.1 in modern, commercially viable process nodes, such as Global Foundries 22nm FD-SOI, TSMC's 22 and 12nm ULL, or Intel's 16nm FF - processes that all utilize low leakage.