I have to say I'm actually impressed by Strava's calorie estimate, at least so far as I just compared. There's a 32-mile route I've ridden dozens of times in the past, and I just did it today again for the first time since I got back from my deployment, and the first time ever with actual power data. I chose a few rides from that historical data, and especially one particular time with very close moving time and average heart rate to today's ride, and Strava's calorie estimate was within around 10% of what I measured today. That's not actually half bad. I haven't compared other rides, or rides with very high heart rate or whatever. I was actually expecting it to be worse than it actually was.