Hi.. I did not have a calorie restriction... just a restriction on what I should and shouldn't be eating. I did not have a liquid diet requirement. I changed my way of eating and lost 50 lbs... twice what my Dr. wanted. But if your dr. requires it then a liquid diet should consist of at least 3 protein shakes a day. Most are low calorie... the one I drink is 30gm of protein, 1grm of sugar and 3gm of carbs and 160 calories. (premier brand at costco)  Drinking the liquids for 2 weeks prior shrinks the liver and makes surgery easier for the Dr. to perform, but does nothing to help you learn how to eat post op. There are a lot of Drs. out there that just do surgery after surgery without euducating their patients on proper eating for the rest of your life. The surgery is just a tool... it is not magic... its effects will diminish if it is not used properly.  I hope you are getting nutritional counseling along with your program. This is as much a mental change as it is a physical one. Without both success  will be difficult.  I don't mean to be harsh or assumptive, but I am just so passionate about this... I don't want any of us to have had this surgery only to be disappointed 2 or 3 years out. 
Good Luck with your liquid diet... it will be tough, but you don't need anywhere near 1800 calories. Getting in the nutrition is more important than the calories!! Best of Luck to you with your surgery... its the first step in an incredible life-long journey!
Take Care,
Debbi

Good Luck!!!!... I'm sure you will do great!! 
Take Care,
Debbi