In the perfect world, a home would consume 100% of its solar production for maximum benefit. In reality, this is almost impossible, and most people will consume around 40% of the solar energy produced. Batteries allow some of the energy that would be otherwise exported or wasted, to be stored at a later date. This can then increase your self-consumption figure, increasing your ROI. With clever timing for when certain devices are used (car charging, tumble dryers etc), some people can get up to 65-75% self consumption of power, which improves their rate of return.