Originally Posted by chasm54

That won't do it for you. It will provide a guide, but conditions - temperature, fatigue and so on - make HR an unreliable method of assessing your calorie consumption. You'll find that different HRMs give widely different estimates of calorie consumption - even different models from the same manufacturer. My old Garmin 305, for example, typically gave a figure about 30% igher than the Garmin 500 I replaced it with, and even the 500 tends to be on the high side.

At my lactate threshold I burn about 15 kcal per minute. So 900kcal per hour is about as hard as I can go - and then only for one hour.