No you do not need redundant capacity. When the sun is up the solar panel for instance simply feeds into the existing grid. When the sun goes down the existing system is still standing by.
You have no clue how the electrical grid works and I don't feel like providing a remedial tutorial to someone who is not likely to listen. This kind of crap is why is impossible to have a sensible discussion on addressing climate change. For such a discussion to be useful people need to be honest and acknowledge the limitations of the technology we have available.