Implicit storage – aka overbuilt and operationally curtailed variable renewable energy (RE) resources – is a synergistic complement to [real] energy storage for transforming these resources from intermittent to firm, at the lowest possible cost. Firm power generation is an indispensable requirement of ultra-high RE penetration when demand must be met 24/365 without reliance on a core of conventional baseload and dispatchable generation. Analyzing data from a large US TSO, we show that implicit storage is by far the most effective strategy to achieve this intermittent-to-firm transformation compared to other strategies that can reduce RE’s intermittency: RE blending, demand flexibility, and geographic dispersion.