Stand alone battery bank?

So I was thinking, and I started to wonder, "Why not just install a battery bank to buy power from the grid at around 3 am?" You wouldn't need a renewable energy system and it would actually start saving you money. Alternatively, you could buy power at around 3 am and sell it at 3 pm. Just for safe measure, is all of that legal? Just want to make sure because you know how regulations sometimes are.