W′ balance, or what a match actually costs
FTP tells you what you can hold. W′ tells you how many times you can go above it.
FTP is a single number describing a steady state, and bike races are not a steady state. They are a threshold effort interrupted by a dozen efforts well above it: a rise, a gap to close, a corner taken badly and paid for on the exit. Riders have always described this as burning matches. The critical power model tells you how big the matchbook is.
Two numbers instead of one
The model splits your capacity in two.
Critical power (CP) is the asymptote — the highest power you could theoretically sustain indefinitely. It sits close to, but not identical with, FTP.
W′ (pronounced “W prime”) is the finite work you can do above CP, measured in kilojoules. It is a tank. Ride above CP and it drains at a rate proportional to how far above you are. Drop below CP and it refills, slowly.
Typical W′ runs about 10–15 kJ for a recreational rider, 15–22 kJ for a trained age-grouper, and 20–28 kJ for competitive riders. What matters is that it is finite and it is yours.
Why the arithmetic is worth doing in real time
The useful part is not the total. It is the balance — how much of the tank is left right now, second by second.
Consider two riders with identical FTP. One has a W′ of 12 kJ, the other 24 kJ. On a flat time trial they finish together. On a crit with eight accelerations out of corners, the first rider is done by lap five and the second is still there at the finish. Same FTP, completely different race.
Watching W′ balance during a ride changes decisions. A 30-second effort at 150 W above CP costs 4.5 kJ. If you have 6 kJ left, you can make it once and it had better be the right move. The model turns “how am I feeling” — which is a poor guide under fatigue, because you feel bad for several minutes after every hard effort regardless of what you have left — into something countable.
Where it stops being reliable
Three honest limits.
The recovery half-life is not universal. W′ refills more slowly the deeper you have gone and the closer you are riding to CP afterwards. Skiba and colleagues’ 2012 formulation models this, but it is a model fitted to a group, and individual recovery kinetics vary widely. If yours differs, the balance will read optimistic late in a hard ride.
It needs a good CP estimate. Get CP wrong by 10 W and the drain rate is wrong everywhere. CP is best estimated from several maximal efforts of different durations — roughly 3, 8 and 12 minutes — not from a single 20-minute test with a percentage applied.
And it assumes a power meter. Estimating W′ from heart rate does not work, because heart rate lags the effort by exactly the interval you are trying to resolve.
The reframe
The value of the model is not the precision. It is that it makes an invisible budget visible. Most amateur riders lose races by spending their W′ in the first third on efforts that did not need to be made, and then having nothing for the one that mattered. Seeing the tank drain in real time is what turns that from a lesson you learn slowly into a decision you make on the road.
Bike Analytics computes CP and W′ balance on the phone, from the ride file your head unit already recorded.