Freezers use energy a different rates depending on the age of the machine, type and total size. There is also a wide variance between manufacturers so shop for the best energy deal that you can find. The average freezer is in operation for over 25 years...more than long enough to make up the initial cost in energy savings.
Energy usage for some Kenmore Freezers:
5.0 cu. ft. Upright Freezer - 321 Kilowatt Hrs per Year
13.7 cu. ft. upright - 621 Kilowatt Hrs per Year
16.7 cu. ft. Upright Freezer - 682 Kilowatt Hrs per Year
20.3 cu. ft. Upright Freezer - 763 Kilowatt Hrs per Year
If you haven't bought a freezer yet then consider a chest freezer. The energy savings can be quite a bit.
5.0 cu. ft. Manual Defrost Chest Freezer - 242 Kilowatt Hrs per Year
13 cu. ft. Chest Freezer - 326 Kilowatt Hrs per Year
Elite 19.7 cu. ft. Chest Freezer - 435 Kilowatt Hrs per Year
19.7 cu. ft. Commercial Chest Freezer - 488 Kilowatt Hrs per Year
To save even more on freezers usage:
Buy a new freezer if yours is more than 10 years old.
Keep the door shut
Keep the freezer in a cool garage rather than in a heated space
Keep the freezer full
Keep an ongoing list of contents on the door so you do not have to rummage around to find stuff.
Keep the compressor coils clear from the wall and clean often.
Turn off the auto-defrost cycle and only turn it back on when it really needs to be defrosted.