Whether it will actually save you money, can depend on a lot of factors, like the size of your home, how often you run the AC and the efficiency of your units. But generally speaking, using a window unit at night should help reduce the load on your central unit, which can in turn help reduce your energy usage and save you some money. If you're looking for more advice or need some help with your AC units, I highly recommend checking out Hot 2 Cold Air Conditioning. They're a great company with lots of experience and knowledge in the field. Good luck!