The calculation consider three factors in optimal conditions, without accounting slope, gravity and special conditions in other aspects. Pace, weight and time serve for a good approximation.
It's interesting to remember that calories are a standard unit to measure energy, and as any other, they can be converted. For instance, converting to watts-hour, anyone could observe how Nature creates a superior standard in energy optimization. A mere kilowatt-hour is equal to ~860.000 calories. Let's analyze after this equality how many calories the human organism burns practicing this great sport.
You can visit the web page with the calculator clicking here.
• 30 minutes, 70 kilograms and 12-14 mph → 280 calories.
• 60 minutes, 100 kilograms and 12-14 mph → 800 calories.
As the aforementioned equality seems really difficult to forget, now it's time to calculate how much time and distance would travel a person with 70 kg at 10-12 mph to consume one kilowatt-hour.
Sadly the calculator breaks with large numbers, but anyway it's easy with its help:
• 122.5 minutes, 70 kilograms and 10-12 mph → 860 calories
Now we need to add three more zeros to achieve the approximately 860.000 calories of 1kW-hour and the result is 122.500 minutes, 2041 hours, or near three months constantly cycling and without sleep! With a pace of 11 mph, the distance is more than 22000 miles / 36000 km.
If anyone is concerned about energy optimization, this looks like a good start.