Downward solar evaporation with multistage configurations is a promising off-grid solution for high-efficiency potable water production. However, a major, yet often overlooked, barrier to practical application is the significant, unquantified performance gap between laboratory benchmarks and field operation, arising from the complex environmental factors. To diagnose this gap, this work first introduces the Environmental Robustness Index (ERI), the ratio of field-to-lab normalized water productivity (P’), as an essential metric. A comprehensive framework is then developed to precisely quantify the effect of key environmental factors, including wind, sky cooling, and ambient temperature, on the ERI. Guided by the framework, we present the spectrally selective air lock strategy as a universal principle to suppress environmental heat losses and improve ERIs. Implementing this strategy significantly enhances downward solar evaporator’s ERIs from 0.55 to 0.98, effectively closing the gap. This study establishes a framework for solar evaporation to move beyond reporting P’lab alone and utilize (P’lab, ERI) as the dual metrics for advancing real-world applicability.