W' (anaerobic work capacity)

W' (pronounced 'W-prime') is the fixed amount of work, measured in kilojoules, that you can perform above Critical Power before exhaustion. Think of it as a battery: every second spent above CP drains it, and time spent below CP recharges it. When W' reaches zero you're done — you have to drop below CP or stop.

W' = (P − CP) × t_to_exhaustion
    (constant effort above CP; W' in joules, P and CP in watts)
    typical range ≈ 10–30 kJ

W' is the second parameter of the critical-power model, alongside CP. It represents a finite, largely anaerobic reserve — glycogen-derived substrate, phosphocreatine, and tolerance for the metabolites that accumulate above CP. Typical values sit around 15–25 kJ (with a broader range roughly 10–30+). Explosive pursuit and sprint-type riders carry a large W', while pure diesel time-triallists tend to have a small one; genetics and training both shape it.

Skiba and colleagues extended the static parameter into a dynamic, real-time model — W'balance — that tracks depletion during hard efforts and reconstitution during easier ones. Reconstitution is neither instantaneous nor linear: it follows a recovery time constant that depends on how far below CP you drop, so the deeper the recovery, the faster W' comes back. This is precisely why interval sessions live or die on the quality of their recovery valves.

W' explains why you can complete, say, 5×3 minutes above CP even though no single 15-minute effort at that power is possible — the recovery windows partly refill the battery between reps. Make those recoveries too short or too intense (still near CP) and W' never recharges, so the final reps collapse. Good coaching back-solves interval targets from W' so the last rep is still completable rather than a guaranteed blow-up.