Update: This calculation is now part of BikeWatts v1.1. Check it out!
I recently received a question from a customer about how to convert the Watts measurement given by BikeWatts to Calories burned. I figured that this might also interesting to others, so below is a high level explanation.
Watts is given in the amount of work per second. Work is measured in Joules (J) – remember that for later – or, in this case, more specifically force times distance.
As for the Calories burned, the value that BikeWatts presents is the amount of mechanical work per second being performed to move you on your bike. Bicyclists are generally 20-25% mechanically efficient, and thus the total energy used by the rider is 4-5 times the amount that is actually transferred to the bike.
Now – back to Joules. It turns out that 1 calorie (small “c”) is 4.184 Joules of work – conveniently right in between that 4-5 times factor mentioned above. That makes the math easy: you can take the number of watts given by BikeWatts and perform a rough conversion directly to calories (again, small “c”) burned per second. The only other thing you need to do is convert to dietary Calories (upper-case “C”), aka. kilocalories.
For example, if BikeWatts says that your output is 200 Watts, you can then estimate calories burned at 200 calories per second. That means that every 5 seconds, you will have burned 1000 calories (i.e. one Calorie/kilocalorie), and every minute you will have burned 12 Calories/kilocalories. That translates to about 720 Calories per hour if you pedal the whole time (no coasting!)