Billions of people are facing freshwater scarcity nowadays, especially those living in remote areas. Thus, decentralized fresh water production is urgently needed. A promising solution is to harvest dew water as it is less dependent on climatic and geographical conditions compared with collecting fog water and cloud seeding. Harvesting dew water can be realized by the method of sorption-based atmospheric water harvesting (SAWH) and passive radiative condensers. A combination of the two methods was demonstrated for decentralized water production. However, since the desired relative humidity (RH) for each method trends in opposite directions, an optimal RH exists for the combined water harvesting system. Herein, we developed a mathematical model to determine the optimal RH under different conditions. A composite adsorbent was used, namely multi-walled carbon-nanotube (MWCNT) embedded zeolite 13X/CaCl₂, with adsorption isotherms that mathematically fit experimental results. P(VdF-HFP) was used as the passive radiative cooler, and the correlation of RH, cloud coverage fraction and the influence on radiative cooling power were mathematically estimated. Various parameters were comprehensively investigated, including adsorption phase time, parasitic heat convection coefficient and climatic conditions. The simulation results show that the optimal RH for SAWH systems using passive radiative condensers is 60%−70% for 200 g MWCNT embedded zeolite 13X/CaCl₂ with an adsorption phase time of 15 mins in the winter of mid-latitude regions. This work provides a guideline for selecting optimal operating conditions of SAWH systems using passive radiative condensers under specific RH and climatic conditions to achieve maximum decentralized water production.