Traditional synthetic refrigerants like R134a are under increasing regulatory pressure due to their environmental impact, hence the need for ecologically friendly alternatives. The present study experimentally investigates and predicts the performance of a natural low-GWP refrigerant, R600a (isobutane), for household refrigeration with R134a. In the study, measurements and modeling of COP, power consumption, and cooling performance were done for capillary tube lengths of 2.43-4.26 m and ambient temperatures between 25 °C and 37 °C. Predictive correlations are developed from the experimental data to estimate system behavior for a wide range of working conditions. The results indicate that while R600a provides a cooling performance close to R134a, it requires a 66% lower charge compared to R134a (50 g vs. 150 g), leading to a reduction in cost and risk of flammability. RR134a delivers better COP values when the capillary tube is short and the operating temperature is relatively low (around 25–30 °C), showing an improvement of about 7.9–24.5%. However, as the capillary length increases and the temperature rises, R600a starts to outperform R134a. At a capillary length of 4.26 m and temperatures between 35–37 °C, the COP improvement with R600a can reach as high as 50.5%. These observations are further supported by predictive analysis, which helps validate the trends and offers reliable guidance for choosing the right refrigerant–capillary combinations across different thermal conditions. Overall, the study offers practical guidelines to optimize the design of household refrigeration systems to strike a balance among efficiency, safety, and environmental sustainability.