Cycling power and kilojoules explained
The difference between watts and kilojoules
Watts describe a rate of doing work: one watt is one joule per second. Kilojoules describe an amount of work. At constant power, multiply watts by seconds and divide by 1,000.
Set the experiment to 200 W for an hour. The rectangle contains 720 kJ. Double duration and it contains twice the work; double power and it also doubles. For a varying trace, add the area of each small time interval. Work helps you compare the total mechanical output of rides that differ in length and intensity.
Constant-power example. The efficiency setting is an assumption, not a measurement or a fuelling target.
The difference between watts and kilojoules
Watts describe a rate of doing work: one watt is one joule per second. Kilojoules describe an amount of work. At constant power, multiply watts by seconds and divide by 1,000.
Set the experiment to 200 W for an hour. The rectangle contains 720 kJ. Double duration and it contains twice the work; double power and it also doubles. For a varying trace, add the area of each small time interval. Work helps you compare the total mechanical output of rides that differ in length and intensity.
Calculating work from a ride
A common mistake is multiplying watts by minutes without converting minutes to seconds. Another is confusing kilojoules of mechanical work with kilocalories of food energy.
Move each control separately. The horizontal axis is minutes, but the calculation converts to seconds. The number you can audit is the area: average watts × duration in seconds / 1,000.
Estimating calories from mechanical work
Only part of metabolic energy becomes external mechanical work. The rest is not visible in a crank power recording. Gross efficiency varies with conditions and workload; a cycling efficiency review explains why choosing one fixed value is an approximation.
The efficiency slider is an assumed value for teaching. Estimated metabolic kcal = mechanical kJ / (efficiency × 4.184). At 25% efficiency, 720 kJ gives about 688 kcal. Changing the assumption changes the estimate without changing measured work.
What your power meter can measure
A power meter can support a work calculation when the recording is complete. It does not measure exactly what you ate, absorbed or need to replace during that ride.
Use the mechanical total to understand the file. Keep any metabolic conversion labelled as an estimate. This experiment is not a fuelling prescription, and a missing recording interval leaves missing work.
Why the area under power is work
One watt is one joule per second. Holding 200 W for 60 seconds therefore produces 12,000 joules, or 12 kJ, of mechanical work at the measurement location. For a changing ride, add the work from each short interval: power × interval duration.
This is why a power-by-time graph is useful: the area under the curve represents the accumulated work. If average power and time are known for the same complete interval, multiplying them gives the same total. Always convert minutes to seconds before calculating.
Why work is not the same as food energy
Your body uses chemical energy to produce force and movement, and not all of that energy appears as external mechanical work. Heat and other physiological processes account for part of the difference. The relationship is commonly described with an efficiency assumption, but it is not a direct reading of calories absorbed or required.
At 25% assumed efficiency, 720 kJ of mechanical work corresponds to about 2,880 kJ of metabolic energy in this simplified calculation, or 688 kcal after dividing by 4.184. Change the assumption and the estimate changes. Individual efficiency, measurement and conditions make an exact conversion inappropriate.
What the total can and cannot tell you
Two rides can have the same total work but very different demands: one may be longer and easier, another shorter and harder, or contain repeated surges. Work also does not say when during a ride the effort occurred. Use the trace and duration alongside the total.
Missing power samples mean missing work unless the platform applies an explicit estimate. The result is not a fuelling prescription: food needs depend on the session, context and the person, not just the work number on a screen.