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The bikes at my gym show your current power output in Watts and I'm trying to figure out what this means in some tangible form.

I know 1 Watt = 1 Joule / second. I know 1 Joule = 0.239 calories.

So for example, if I average 200 Watts over an hour ride, what does this mean.

Does it mean I pushed 200 Watts for the whole 60 minutes burning a total (of 200 * 0.239) = 47.8 calories (seems very low!)

or was I pushing 200 Watts every second giving me (200 Watts * 0.239 calories per Watt) * 60 seconds * 60 Minutes = 172080 calories per hour (seem ridiculous)

or was it 200 Watts per minute (200 Watts * 0.239) * 60 Minutes = 2868 calories per hour which still seems really high.

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3 Answers 3

up vote 3 down vote accepted

Generally, given the mechanical efficiency compared to human efficiency, you can multiply your watts x 4 and get your calories burned. It's going to vary some, as some people are more efficient than others, but that is a good way to get pretty close. It's based on both human and mechanical efficiency of the bike being around 20-25%.

The math:

200 watts at the powermeter x 3600 (seconds in an hour) = 720,000 joules = 720 kJ (kiloJoules)

1 kCal (A food Calorie is actually 1000 calories, small c, or 1 kCal) = 4.18 kJ

So 720 / 4.18 = 172.24 kCal

Now, take that and divide by the efficiency factor, which is around 19-26% depending on the person. Take the middle, 23%, and you get:

172.24 / .23 = 748.9 kCals for your hour of work, which is right around 4*200. You may burn slightly less or slightly more depending on your own efficiency.

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I calculated a bit and your second calculation should be right. It only seems ridiculous, because you've got 172080 calories, which is 172.08 kcal. It still is really low for an hour of biking, though.

(Some notes on my calculation: 200 Watts for 1h is 0.2kwH = 0,72 MJ = 172 kcal)

You might want to use this calculator for future calculations, as it's quite easy to use and just much faster.

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It's not low, it's just the mechanical output. Total energy consumed it considerably higher, just that the vast majority (75%+) of it is waste heat, so multiply by 4-5 to get the total calories/kilojoules used. –  Compro01 Mar 14 at 14:38
    
@Compro01: I suspected something like that, thanks for clarifying. –  LarissaGodzilla Mar 16 at 15:57

JohnP gave a good explanation of the calculations involved.

Note that wattages and calories on exercise equipment are generally inflated, and often inflated significantly. 200 watts/hour is a pretty hard effort; I ride about 3000 miles/year, and my best hour effort for last year was about 215 watts.

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