If you know duration (time spent running) and you have the average power during that time, you can quite easily calculate energy.
Power(Watts) = energy(Joules) / time(seconds)
We can figure out our energy using the equation above, but unfortunately, it’s in joules. We need to get to calories so we use the following conversion:
1 Joule = 0.238902957619 calories
Note, that what we commonly call a calorie, is actually a kcal or Calorie. So we need to divide by a factor of 1000 to find our common usage kcal (when you look at a nutrition label those calories are in fact kcals).
There are two types of calories: A small calorie (symbol: cal) – 1cal is the amount of energy required to raise one gram of water by one degree Celsius. A large Calorie (symbol: Cal, kcal) – 1Cal is the amount of energy required to raise one kilogram of water by one degree Celsius.
We also need to account for human inefficiency. Like any machine the human body is far from perfectly efficient – it has to burn more than 1 joule of real energy to output 1 joule of measured energy through a power meter. We assume the efficiency of human runners to be about 24% (so your body’s many systems and inefficiencies burn 4J of energy for every 1J you deliver to the pedals).
By coincidence, a joule is 0.000239006 calories, or if we multiply by 1000 to get kilocalories approximately 0.24. How convenient!
Take all the above into account and we can build a very simple equation for calculating calories burned while running from average wattage:
energy (kcal) = avg power (Watts) X duration (hours) X 3.6
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