To start saving on the electricity that your refrigerator uses, you'll need to gather some cheap materials. If you buy milk, water or vinegar in gallon bottles, you can recycle the bottles to store cold energy in the refrigerator. However, if you have smaller plastic bottles, these can be very effective for filling up small gaps of unused space inside the refrigerator and freezer. If you're careful, you can also use glass bottles filled with water to store cold energy in your refrigerator.
If you like, you could also get some inexpensive gel-filled ice packs to fill in small areas of empty space. These ice packs are generally available at your local dollar store or supermarket for low prices. If you're using bottles, rinse them out and then fill them with water. This method is very similar to keeping the refrigerator as full of food as possible, but it's easier and less expensive. When the refrigerator door is opened, less cold energy will be wasted because the cold is being stored in water rather than in air. Also, your food will stay cold longer in the event that the electric power fails. If you need the space to store food again, simply remove the cold pack from the refrigerator.