W': anaerobic work capacity

The total anaerobic energy available above critical power, expressed in joules.

In the Critical Power model, W' (pronounced 'W prime') represents the total anaerobic energy available. Any power above CP depletes W' at a rate proportional to the excess over CP. Once W' is empty, sustaining power above CP is impossible. W' recharges below CP, but slowly. In Ironman racing, deliberate W' depletion is rare, but an aggressive climb or too-fast start can draw from it. Knowing W' helps manage surges and hard passages.