The energy dependence of radiation damage in silicon for proton energies in the range 1.35 to 130 MeV has been measured by observing the degradation of the bulk minority carrier diffusion length in silicon solar cells. Variability in proton flux determination at four different accelerators was minimized by employing prebombarded solar cells with known minority carrier diffusion lengths as calibrated solid‐state ionization chambers. Where beam intensity measurement comparisons with Faraday cups could be made, agreement to better than 5% was obtained.The quantity characterizing the damage rate is the rate of change of the inverse square diffusion length with fluxK≡d(1/L2)/d&PHgr;. The 1‐&OHgr;‐cmp‐type silicon degraded, on the average at a rate six times less rapid than 1‐&OHgr;‐cmntype, independent of energy. Room temperature annealing gave 30% to 50% decrease inKwhenever the diffusion length was measured during and after irradiation. The energy variation ofKagrees with the variation predicted by Rutherford scattering below 8 MeV, but decreases less rapidly at higher energies.The measured diffusion lengths increased with excess carrier densitynfrom 2% per decade atn=109cm−3to 20% per decade atn=1014cm−3. The reported results, obtained at low excess carrier density, can be used to predict solar cell degradation under conditions of outer space illumination if the appropriate excess carrier density is used. Failure to take into account the diffusion length variation will result in an underestimate of the solar cell output of less than 7%.