Spark's calorie calculator assumes that calorie burn is proportionate to body weight. This holds true for many forms of exercise (eg. running, walking, aerobics), but for things like a recumbent bike, you are working against the resistance of the machine, rather than your bodyweight. This can lead Spark to overestimate the calorie burn for stationary cycling, especially for heavier people.
If you were using a low resistance setting, 300 sounds more reasonable. 800 calories is about the limit of what a very fit person working extremely hard may be able to burn in 1 hour - figures above this range should be regarded with great suspicion. 300-600 cal/hour is more realistic for most people.
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You should try to estimate calorie burn based on how hard you were working in this instance. If you were pedaling at a high resistance, going all out, sweating and breathing hard for an hour with a high heart rate, maybe it could get to 800 but it would be very difficult. However if you were going at a more moderate resistance, heart rate and breathing moderately such as maybe taking a fast walk, it is more likely to be around 400 in an hour.
I tend to go with the lower of the two numbers - seems a better idea than overestimating calories burned. The piece of fitness equipment also knows what resistance you've set throughout the ride, where the SP numbers are an average and would include things like wind resistance as if you were biking outside.
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