There’s a simple formula
All you need to know is the time spent cycling and the average power during that time, and you can get the energy burned using this general formula.
Energy (Joules) = Power (Watts) * Time (Seconds)
The problem is that you want to know your energy expenditure in kcal, not joules. Note that what is commonly referred to as calories is in fact kilocalories, that’s why the abbreviation says kcal, not Cal. So, we would use a conversion rate of 1 Joule = 0.238902957619 kcal.
Human efficiency is only 24%
Another problem is that humans aren’t perfectly efficient engines. We spend a lot of energy on heat production, balance and other things while riding a bike. In fact, efficiency of cycling humans is around 25%. We burn 5 joules of energy for each joule delivered to the pedals. That means we should divide the measured joules by 0.25 to calculate the actual expenditure. By coincidence, the joule to kcal conversion mentioned above is 0.2389. So, the last two problems cancel each other out, and we can just swap joules for kcal.
How to calculate it
There’s just one last adjustment to make. The fact that we multiplied calories by 1,000 to get our desired kilocalories (kcal) means we also need to adjust time. We will multiply it by 3.6 to get hours. This is the final formula you can use. Just multiply your average power output on the bike by the number of hours you spent riding times 3.6 and there you have it – the number of calories burned.
Energy (kcal) = Power (watts) * Time (hours) * 3.6
Example: One hour at 200W
We can now easily calculate the answer to our initial question of how many calories you burn when you average 200W during an hour of riding.
200 watts * 1 hour * 3.6 = 720 kcal
In contrast, Chris Froome can sustain 414 watts for 30 minutes, which means he would burn more in half an hour than a relatively fit average cyclist in double the time.
414 watts * 0.5 hours * 3.6 = 745 kcal
Of course, not even these figures are perfect, especially as the human inefficiency factor of 24% is just an estimate. But we can say that these caloric values are accurate plus or minus 4%, which is still a lot better than most calculators and apps that can be off by 30% and often by a lot more.