Following basic units of power conversion, I’ve calculated that 200 watts for one hour equals ~177 calories (i.e. food calories = kilocalories). You can verify this with many online calculators like this one:
However, my Polar watch and other cycling references seem to say that averaging 200 watts for one hour while cycling burns much more than 177 calories (probably closer to 1,500 calories or so). I’m guessing this has something to do with the fact that the human body can not generate power with 100% efficiency - that is, to generate 200 watts of pedal power actually takes much more energy (calories).
Is this right? If so, does anyone know what that efficiency ratio is supposed to be?
Or, more simply, given an average watts per hour (or second or whatever) what’s a better formula to determine how many calories someone burned?
Its also easier if you convert watt*h to kilojoules and just take that number. Considering other unit conversions and the overall system efficiency, you’ll have about the right calorie number (but it won’t technically be in the right units). If you use a Powertap you’ll learn this rather quickly since the output is in kJ.
So, taking your 200W, at 1 hour, gives 720 kilojoules. Take ~720 food calories as your rough energy expenditure. 172kcal would be the energy output (or for a slightly more entertaining value, you’re making 0.0001721 tons of TNT)
1 kilocarlorie or 1K - a food calorie is approx. 4186 J. Further cycling efficiency in humans is in the range of 20-25% for most people with 25% being excellent and not that common. Normally one should be somewhere at 22-23% after a few years of endurance training. A good working assumption will be 22.5%.
At 200W at the crank, you have 200 W x 3600 sec. = 720 000 J/4186 = 172 Kcal/hr. Then using the 22.5% efficiency assumption we’ve got 172 Kcal/.225 = 764.45 Kcal (food cal.) actual metabolic cost of cycling at 200W output. If you are new to the activity and don’t have any natural gift for this thing, it would be more like 172 Kcal/.2 = 860 Kcal/hr. On the other hand, if you do have world class cycling efficiency, it would be something like 172 Kcal/.25 = 688 Kcal.
If your HR monitor shows something drastically different, make sure you feed it realistic assumptions for resting HR and weight (i.e try to use “lean” weight instead of total weight). Still sometimes these gadgets are not good at all at even guessing the ballpark of actual caloric expenditure. The counter could still be used as a rough indication of a workout’s energy cost as it will at least be consistent, if not accurate.
