Power
W' (W-prime)
The amount of work, in kilojoules, you can do above Critical Power before exhaustion.
W' (read "W-prime") is the amount of work, measured in kilojoules, that you can perform above Critical Power before reaching exhaustion. Think of it as your finite anaerobic battery.
How it is calculated
W' is derived from the Critical Power model. By riding two maximal efforts of different durations (e.g. 3 minutes and 12 minutes) you can solve a two-equation system for CP and W'.
Power(t) = CP + W'/t
Typical W' values fall between 8 and 25 kJ for trained cyclists.
Why it matters
W' is the budget for hard surges. Once you have spent it (sustained efforts well above CP), it is gone until you recover. Models like W'bal track how much of your W' is left in real time during a ride.
Common misconceptions
- W' is not the same as VO2 max. Different system, different unit.
- W' recharges with rest below CP, but not instantly — full recovery takes minutes, not seconds.
- A bigger W' is not always better — for long endurance racing, CP matters more.
Related concepts
CP, FTP, VO2 max.