I use random fluctuations in hourly wind and solar generation in California to estimate how much they reduce emissions of carbon dioxide, sulfur dioxide, and nitrogen oxides. These offsets depend on the direct displacement of high-cost natural gas generators, and on the hydropower reallocation that occurs to the hours with the lowest increase in renewable generation. Solar power daily intermittency shifts hydro from the afternoon to the evening, which increases its emissions offsets since the gas generators displaced in the evening are dirtier than those kept running in the afternoon. In contrast, wind offsets are less sensitive to hydropower reallocation, since it leads to a substitution of generators with similar emissions intensities. These findings highlight the importance of accounting for interactions between wind, solar, and hydro capacity in assessing their environmental benefits. Similar lessons will apply to electric grids with storage capacity.