It works a little better with a Garmin. Also since my Garmin Edge 500 has a pressure altimeter, the elevation data is more precise, though no more accurate, than elevation given by any smartphone app. Precision is important as it gives better vertical speed data. I did some simple calculations on a climb I did hauling 250 pounds (me and bike) up a a 24 foot rise in 13 seconds, and came up with 600 Watts. Strava told me it was over 1k Watt. I would really like to believe Strava, but no. . . gotta keep it real.

I suspect that the way Strava calculates calories is more straightforward, therefore more accurate (HR*time*coefficient from table for weight and BMI). Power is more difficult in that air resistance, especially over 15 mph is a big factor in how much power is being put out, and Strava assumes that all riders have the same drag coefficient and cross section area, which they most certainly do not.

I do use the calories figure in my diet planning. The calorie counter I use is called FatSecret, and you can enter "activities" like "biking easy-10mph, moderate-13 mph, fast-15mph and very fast-17mph".

On a 20 mile, 800 ft climbing, ride that I can do in about 1'45" (about 15mph average), the calories Strava shows is about 50% lower than FatSecret gives for the equivalent time riding

*easy*.

The Strava number for calories is about 10% higher than the figure calculated by my Garmin, so I split the difference and use that. I enter however many minutes at "easy" will give me the equivalent calories in FatSecret. If I am consistent and eat 80% of that total, I can drop 1-2 lb/week.