Another quickie.. HR vs Watts

What an eye-opener this Powertap device is. Took my first ride home from work with it yesterday, at the CORRECT "active recovery" pace. Seems that I haven't really done a proper recovery ride in the past three years! Typically, my goal on recovery rides was to simply keep the average heart rate in the 135-140 range. All the while I was in "endurance" mode.. Last night riding home from work, I averaged 125 watts and 114 Avg HR. And this morning, more evidence that HR is not as useful a measure as we thought: Ride to work this AM was 118 watts, 121 avg HR. Why? Maybe because of the tail wind? Maybe because it was 20 degrees cooler out than last night? Morning vs evening? Most likely, riding to work is more stressful than riding home, me thinks.