A calorie is the energy needed to raise the temperature of 1 gram of water through 1 °C (now usually defined as 4.1868 joules). When nutritionists talk about calories they usually mean Kilocalories (1000 Calories – Also sometimes called large calorie)
The basic unit of work measured by power meters is the Joule or kiloJoule (1000 Joules) which is approximately equal to 1 Kcal. This makes it very easy to calculate nutrition requirements when cycling with a power meter.
The actual conversation rate is 4.18kJs = 1 kcal, but most people range from 20-25 percent efficiency. This means that for every 100 calories burned in exercise, only around 25 at best propels you forward and are measured at the power meter. The rest is turned into heat. If you know your power output and time, you can calculate calories burned.