VirtualTraining records energy as work and everywhere in apps you can see energy in kJ's only. If you want to convert energy into kCals, see below.
Work or Energy (Joules) = Power (Watts) x Time (Seconds)
There are a number of different ways to represent energy. A kilojoule is a mechanical representation of energy. In our everyday world, however, we typically represent energy, thermally, as the amount of heat released when burning a quantity of food. Thus, we normally think of energy in terms of the amount of food we can eat in kilocalories.
To get an idea of how many Kcals you burn for a given number of Kjoules of energy transferred to the bicycle, you need to know that 1 Kcal is equal to roughly 4 Kjoules (4.186). So if you do 1000 Kjoules on the bicycle, you’ve really transferred about 250 Kcals of energy to the rear hub. But that doesn’t mean that you’ve burned 250 Kcals worth of food. This is because while riding a bicycle, the average person is only about 22% efficient. That means if you burn 1000 Kcals of food while riding a bicycle, only about 220 gets transferred to the hub to make the bicycle move. The rest just gets wasted as excess heat. So by a quirk of nature, 1000 Kjoules measured by the Power Tap is equal to just over 1000 Kcals burned by your body. For an average U.S. pro the conversion of Kjoules to Kcals works out to be about 1.1 Kcals for every Kjoule (Table below).
During a typical 5 hour stage in the Tour De France the average rider can do close to 4000 Kjoules of work and burn about 4000 Kcals worth of energy. In contrast, the Surgeon General currently recommends that the average American accumulate at least 1500 Kcals worth of exercise each week to maintain a healthy lifestyle. If you want to ride in the Tour, try and do 4000 Kjoules in 5 hours. If you want to stay healthy, just try and do 1500 Kjoules over the course of a week.
In training and racing, the total energy used by a cyclist has become our primary or gold standard measure for their training volume. In the past, distance or miles was a cyclist’s primary way of measuring volume. Later, there was a greater focus on time rather than distance. Now we focus on a combination of both time and intensity or the total energy or work. Intuitively, we know that it’s not so much about the time it’s about the quality of that time, which is directly reflected in the work done.
The conversion of Kjoules to Kcals depends on a person’s gross mechanical efficiency (GME) or the percent of food energy a person is able to convert to actual mechanical work. For most individuals GME is equal to 22%. There is evidence, however, that different factors like training state, biomechanics, temperature, type of ride and a cyclist’s general experience level can impact their GME. Thus, the number of Kcals burned for a given number of Kjoules can vary from a high of 1.33 Kcals per Kjoule to a low of 0.92 Kcals per Kjoule depending on the individual and condition and is not necessarily dependent on a rider’s experience.