Good question.

Power calculations are a better indicator of energy expenditure than HR monitor because energy output and HR are not linearly correlated. Here is how to calculate energy expenditure using power :

Energy = power output x time.

Power is measured in watts (W) and time in seconds, which gives an answer in joules (j). 1cal = 4.19 j,

Example : 500 kj would be 119 kcal. however, the body is about 20 - 25% efficient, which means that you expend 4 - 5 times the energy, i.e., ~ 500 kcal. So basically in the end to get the number of calories spent in one hour, just take your average wattage for an hour and multiply by 3600(seconds in one hour) then divide by 1000 (to get the number in kilo calories).

By the way, the best way to measure power output is with a power meter like PowerTap. I use a spreadsheet but it only gives an average for a climb and not instantaneous values. I envy people who can dish out over 1k for a PowerTap hub.