Electricity power has served as an essential source in our daily life. However, some remote areas that are difficult to be covered by the power grid, are still facing a serious shortage of electricity for outdoor equipment such as field monitors. Off-grid power is the alternative power in such areas, but there arise apparently economic and environmental problems. Therefore, the development of portable, pollution-free and sustainable power supply equipment has vital research significance. In this paper, based on the radiative cooling and greenhouse effects, a passive thermoelectric system without any active energy input is proposed. A square copper plate coated with a thin film of acrylic acid doped with SiO₂ particles, with an average emissivity value of 0.937, is selected as a radiative cooling material. The commercial polyolefin film with a thickness of 0.12 mm is selected as a greenhouse material. The radiative cooling effect cools the cold end of the thermoelectric generator (TEG) during the nighttime, the greenhouse effect during the daytime is utilized to increase the temperature of the hot end of the TEG. The radiative cooling effect and the greenhouse effect both result in the increase of the temperature difference between the cold and hot ends, and thus obtaining the output power. During the period of time from June 17 to June 21, 2020, the performance of the designed system at the location of Shaanxi, China was evaluated experimentally, and the weather condition effects were also studied. The experimental results show that a stable temperature drop of ～1.1 ℃ of the cold end is achieved via the radiative cooling effect at night. Owing to the greenhouse effect, the temperature increase of the hot end reaches a maximum value of 13.9 ℃. When the average ambient humidity decreases from 45% to 20%, the average temperature difference between the hot end and cold end of the thermoelectric module increased from 1.6 to 1.9 ℃ throughout the day, and the average power increased from 47.8 to 67.3 mW/m², indicating that the equipment can have better power generation performance under the condition of 20% ambient humidity. The device developed in this work realizes all-day passive output and shows that it has potential applications in off-grid power supplies.