The electrons and protons of the Van Allen Belt cause radiation damage to silicon solar cells which results in a gradual reduction of the power output of solar power plants on satellites passing through the Belt. The study of the radiation damage caused by monoenergetic electrons and by monoenergetic protons of various energy indicates that N on P solar cells are substantially more radiation resistant than the commonly used P on N cells. From such studies and a knowledge of the composition of the Van Allen Belt, a flux of, for example, 1 Mev electrons can be determined that will cause the same radiation damage per unit time to a bare solar cell as the Van Allen Belt spectrum would produce under a given shielding. Observations of radiation damage on satellites are also conveniently expressed in terms of such an equivalent flux. Measurements of solar cell short circuit current changes and changes in the current gain of specially designed highly radiation sensitive transistors have been performed on the Telstar satellite. The results are in general agreement with the radiation damage expected from the observed particle flux and they indicate that solar power is practical for long-life satellites passing through the Van Allen Belt, particularly if N on P solar cells are used.