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I have recently purchased a solar powered gable fan for my attic. This fan will only work when the sun is up, but I would like a little more out of it during the early evening hours (no sun). Is there a solution where I can purchase a battery that will be charged by this solar panel, while working the fan, and get a little more juice during the evening hours? If so, how can I calculate which battery I need and where to purchase? Thanks in advance for your answers.