Temperatures below 1 kelvin are highly beneficial, if not prerequisite, to several important technologies that are key to development in present and upcoming decades. Examples include superconducting electronics such as x-ray calorimeters, qubits, single-photon detectors and RF amplifiers. In spite of the typically small size of the elements to be refrigerated, the techniques commonly used to access sub-kelvin temperatures are expensive and cumbersome, due to intrinsic need of circulating the rare 3He cryogen or the heavy magnets required for their refrigeration. These limitations have been an obstacle to broad-scale deployment of sub-kelvin electronics and photonics. Here we develop an extremely compact and fully electrical general purpose solid-state refrigerator, able to continuously cool electronic and photonic devices from above 1 K to below 100 mK, without any need of thermodynamic cycles based on 3He cryogenic fluid or magnetic fields. Our approach is based on a recent technological discovery which showed that superconductive tunnel junctions can in fact operate similarly as vacuum isolated thermionic coolers. This approach provides full scalability for the total cooling power and ability to create large temperature drops with cryogenic electrical coolers - features not available before. We follow this approach and engineer a 3D stacked multi-chip cooler system. We capitalize on several of our previous milestones and aim to demonstrate a complete cooler system that can reach performance comparable with dilution refrigeration, without need of 3He and at a fraction of the mass and cost. Our vision entails new application avenues in the fields of quantum technology, material analysis and surveying, radiation detection, cosmology, and astronomy. We expect significant impact for airborne or space-oriented applications, because of the breakthrough reduction in payload mass and complexity allowed by our cooling solution.