The hybrid power system (HPS) is an application variant of distributed generation, defined as a system utilising two or more energy sources, such that the power generation is more efficient, reliable and cost-effective, offering a better option than single-source systems. HPSs can be used in urban, rural and remote areas. HPS research has focused on sizing and optimisation, which requires efficient and effective methodologies to ensure reliable power supply and a cost-effective system. This paper presents a mathematical programming technique for the design of off-grid HPSs, taking into account uncertainties in renewable energy resources. The basic model formulation is based on a comprehensive superstructure that includes all possible connections for power allocation. Chance-constrained programming is applied in determining the optimal capacities of power generation and energy storage with a specified system reliability level. A case study is presented to demonstrate the application of the proposed approach.