This is the best bike calorie calculator I have used:

http://bikecalculator.com/wattsUS.html

I received the link here at Bike Forums a couple years ago. It's WAY less than those online calculators. People with Power systems say that it is very accurate and comparable to their outputs.

I'm 200 pounds (roughly) and on three-hour 16mph rides I burn around 1200 calories. As you see it's quite a bit less than the 1000 calories per hour figure!