Battery is available practically from the beginning, right after getting Solar Cell. It can come in handy when selling power automatically isn`t quite up to snuff yet, by expanding the pool of stored energy. Stored energy is immediately available to sale, both to offices and to player via selling button.
However, any design that doesn`t sell all the power it makes will eventually start losing energy (and thus, money) when the batteries top off. So while use of batteries allows player to take longer before selling power manually, it`s not a true alternative to autoselling through offices.
Battery takes no heat and gives no heat, which makes it a good insulator for heat-sensitive structures like offices and research centers. But then again, a empty space would do just as good. Though with the price of 50, battery is not something regular player has to weigh carefully.