Agreed as to over estimating calories burnt, typically 5 to 7 calories per minute, during relaxed casual walking vs under estimating calories, typically 1.6 calories per minute, when doing intense spinning. I spin in the 150 to 175 watt range measured with calibrated electronic equipment. I am an electronics engineer so I know how to measure this stuff.
Calculation shown below is from "gearandgrit<dot>com". The muscle efficiency of a human can vary quite a bit depending upon activity, age, health conditions etc. so calorie counts may not accurately reflect true burn rate. What can be accurately measured is your output power. Think of your spin bike as an engine dynamometer setup to measure leg power instead of engine power. I track on an Excel spreadsheet the power that is generated by my spin bike.
From Watts to Calories
If you know duration (time spent cycling) and you have 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) – 1 cal is the amount of energy required to raise one gram of water by one degree Celsius. A large Calorie (symbol: Cal, kcal) – 1 Cal 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. The efficiency of cycling humans is between 20-25% (so your body’s many systems and inefficiencies burn 4-5J of energy for every 1J you deliver to the pedals).
This means we should divide any measured joules figure by 0.2 to 0.25 to find out our actual expenditure.
By coincidence, a joule is 0.000239006 calories, or if we multiply by 1000 to get kilocalories approximately 0.24. How convenient!
So, assuming a human with 24% efficiency, you can cancel those last two steps exactly, and assume that measured joules = real calories burned.
Take all the above into account and we can build a very simple equation for calculating calories burned while cycling from average wattage:
energy (kcal) = avg power (Watts) X duration (hours) X 3.6
You might be wondering where that 3.6 came from. It’s simply an adjust for time and the factor of 1000 reduction to get from cal to kcal (60 seconds/minute * 60 minutes/hour * 1/1000 cal/kcal = 3.6).
