Construction of solar panels is required to meet international standard IEC 61215. In this standard the requirement is that all solar panels should withstand a hail stone of diameter up to 5 inches at terminal velocity without sustaining damage.
Though some panels, particularly the higher quality ones may be able to withstand more than this it should be assumed that any hail of greater than inch in diameter will cause at least minor damage to your panels.
At best, if your panels are damaged by hail it will cause some fracturing on the surface of the panel reducing the efficiency of that panel. At worst, the hailstone could entirely fracture a large portion of the panel rendering it unusable and in need of replacement.
In summary, while hail could damage a solar panel, the likelihood is very small and occurrences are extremely rare.
