A hard effort can change what is left for the next one. W′ balance follows the modeled work capacity available above critical power through a recorded ride, showing its decline during hard efforts and recovery between them.
Screenshots show a synthetic ride calculated and rendered by EffortSignal. W′ balance is included in Free and Pro when compatible power data and model inputs are available.
Find W′ balance in the Power tab
Open a completed cycling activity and select Power. When a saved W′ balance result is available, its chart appears after the power timelines. The line uses the same time axis, zoom and pause-display controls as the other charts.
Read capacity, recovery and negative values
W′, pronounced “W prime”, is a modeled amount of work above critical power, expressed in joules or kilojoules. CP is the fitted critical power in watts. Above CP, the balance falls; below CP, it recovers. At CP it stays unchanged. Every ride starts at 100% in this model.
Hover over the line to read both kJ and %. The saved CP and W′ remain beside the chart. The scale covers 0–100% and extends below zero when needed, so a small change is not enlarged into a full-scale drop.
The line is neutral above 20%, orange from 0 to 20%, and red below zero. These colors help you read the chart; 20% is not a physiological threshold. A negative balance means the recording exceeded this combination of CP, W′ and recovery assumptions. It does not mean your body had negative energy.
What happens during one effort?
With CP 280 W and W′ 24 kJ, two minutes at 400 W use 14.4 kJ: 120 W above CP multiplied by 120 seconds. Starting full, the balance falls to 9.6 kJ, or 40%.
Two minutes at 160 W then restore it to approximately 16.1 kJ, or 67.1%. The next effort begins with less modeled capacity than the first. The example illustrates the calculation; it is not a prescription for work or recovery duration.
Where CP and W′ come from
EffortSignal fits CP and W′ together from recorded cycling power-duration efforts. Current estimates combine indoor and outdoor rides and available recorder sources within a 180-day window, using the strongest supported 5, 10, 20 and 30-minute efforts. Device estimates are not averaged.
Ordinary recordings do not prove maximal effort, so confidence in the current estimate is capped at Medium. A single long-effort estimate without a fitted W′ cannot supply this chart. Your configured FTP and the final eFTP value are not substitutes for CP.
Each saved chart retains the CP and W′ used to calculate it. Later estimates, an FTP edit or power smoothing do not rewrite that result. Earlier charts can retain inputs from an older, device-specific estimate.
Pauses and missing power affect the model
The calculation follows elapsed time, including stops. Confirmed timer pauses, missing or invalid power, and long recording gaps are treated as 0 W and allow recovery. A sensor dropout can therefore overestimate recovery. Hiding pauses changes the displayed time axis, not the saved calculation.
The model starts each ride full and does not carry depletion over from a previous activity. Power-meter accuracy, the efforts in your history and the recovery assumptions all affect the result. W′ balance is not a measurement of energy stores, Garmin Stamina or your readiness to train.
Why the chart may be absent
A power recording alone is not enough: the activity needs a completed, compatible W′ calculation with positive CP and W′. Without that saved result, the chart is omitted. Its absence is not a zero-capacity reading. Earlier activities are not automatically given a W′ chart when a new estimate becomes available.
Put the line in context
Review the recorded power and the duration of each effort alongside the balance. Use interval analysis to inspect the work and recovery periods, and power-duration curves to see the efforts behind your cycling history.
About the calculation
EffortSignal uses the differential form of the W′ balance model, with exponential recovery below CP. Research describes both integral and differential formulations with different assumptions. The linked papers explain that background; they do not validate EffortSignal’s estimates or its treatment of missing power.
- Skiba et al.: recovery of work capacity above critical power
- Skiba and Clarke: mathematical and methodological considerations
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