Depends, is that 15kg of ‘fat’ or 15kg of lippid. The fat cells in your body are 72%-87% lippids (i.e. actual fat like cooking oil), the rest is the cells structure and water.
Then there’s an argument about how much energy there is in ‘fat’ because the body uses up quite a bit in trying to burn it IIRC it’s 8.7kcal of usefull energy, 9.5kcal in total. So if you eat 1g of fat, 9.5kcal, which is then stored as about 1.3g of fatty tissue, then you only have to burn off 8.7kcal to lose it again.
So to lose 1kg of fatty tissue the actual number of calories is closer to 7200kcal.