The emerging field of soft actuators is anticipated to significantly impact areas that require enhanced adaptability and safe interaction including biomedical robotics. The electro-ribbon actuator, characterised by its compliance, lightweight, high efficiency, extensive scalability, and direct electrical control, emerges as a promising candidate for the future of soft robotics. Although the electro-ribbon actuator demonstrates efficient performance, a comprehensive understanding of its fundamental mechanics remains unexplored. To explain the impact of the actuation mechanism and the design parameters on the performance of the actuator, we have developed a mathematical model grounded in large deformation beam theory. As evidenced by our experimental results, this model accurately predicts the quasi-static behaviour of electro-ribbon actuators. Utilising this model, we establish a strategic road map for the development of electro-ribbon actuators with a range of passive stiffness tailored to achieve required output forces and displacement. This study provides a pivotal foundation for the model-based control and design optimisation of electro-ribbon actuators.