[InnocentB]

Well a calorie is the amount of energy to raise a gram of water by one degree Celsius, so if you're drinking 2 litres of water a day and it's 0 degrees vs 20 degrees, you'll burn around 20,000 calories a day from the difference.

Note though that that's calories, and not kilocalories, which is what we generally talk about for dietary purposes. So ~20kcal a day, which would represent about a 1% change for the average person. Not a huge gain.

[eru]

Also, people might just eat a spoonful of more food, if they need 20kcal more per day.