Yeah you do need to eat a fair bit more than that. The reason for this is that your metabolism will slow as your body wants to adjust for your survival. You'll lose muscle and water instead of fat, and then the weight-loss will pretty much stop when your body has adjusted to its new minimal amount of calories.
This also means that if you have any naughty days, or stop trying to lose weight, that you're much more likely to put it back on because your body's new basal metabolic rate is really low and very easy to exceed.
The best approach would be to reduce your calories some, but not quite so much, and keep an adequate intake of complex carbohydrates - and protein, which is probably in your meal replacements. Also, cardio! Don't forget your exercise, this is what will burn your fat.
Oh! and it is definately realistic to lose 20kg in a year.