Often, when someone buys a new refrigerator, they will take the old working fridge, move it to the garage and leave it plugged in. That's a huge waste of energy. Keep it unplugged unless you actually need it.
Photo by Nicolas Nova
The Washington Post points out your older fridge probably isn't as energy efficient as newer models, so running it constantly can be expensive. . The article suggests getting a refrigerator large enough to hold all your food and drink, since one unit is far more efficient than two.
If you need some extra cold storage space, especially around the holidays, you can keep that extra fridge. Plug it in a few days before you need it and unplug it when you aren't using it. (Remember to leave the door open when it's not powered to avoid unpleasant smells.)