A battery will allow you to store any electricity you don't use, so that instead of exporting the energy back to the grid, you can use it later when the sun isn't shining. The initial cost for a battery is higher than solar panels alone, but it's a great idea if you wish to be self-sufficient and avoid spending money on pricey electricity from the grid in the long-run. You can also guarantee that the majority of the electricity you're using is 100% clean, green and home-generated.