I don't have a power meter but I do notice that I, more often than none, get varying results in calories burned across different sites. I upload to Strava, RWGPS and Endomondo and usually get three different results for the same ride. I always go with what my Bolt and Tickr record.
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HCFR Cycling Team
Ride Safe ... Ride Hard ... Ride Daily
2017 Colnago C-RS
2012 Colnago Ace
2010 Giant Cypress