The efficiency of a solar cell is the percentage of sunlight energy that can be converted into usable electrical energy. The efficiency of a typical commercial solar panel varies between 15% to 20%, while some panels can achieve up to 22% efficiency. Factors like the type of material used, cell design, and environmental conditions can affect the overall efficiency. Increasing the efficiency of solar cells is an ongoing research area, with scientists working on new materials and designs to achieve higher conversion rates.