Critical Power and W′: what it is and what you can do with it
Almost all training advice revolves around a single number: your FTP. But a race isn't decided by how hard you can ride for an hour — it's decided by how often you can go above it, and how quickly you recover from that. This piece explains the concepts that actually describe that.
Critical Power — your endurance engine
There's a relationship between how hard you ride and how long you can sustain it: the higher the power, the shorter you last. Critical Power (CP) is the power your endurance engine can keep delivering — theoretically for a very long time, because your body stays in balance. Ask for more than your engine delivers and that balance is disturbed, and you eventually run empty. In short: above your CP, the time you can hold it is limited. (Monod & Scherrer, 1965; Jones & Vanhatalo, 2017)
CP resembles FTP, but isn't the same. FTP is a definition: the wattage you can hold for an hour. CP is about the power-duration relationship itself — with it you predict how long you can hold a given wattage, whether that's 3, 5 or 20 minutes. And the real difference: CP also has an energy value attached to it, namely W′.
W′ — your attack battery
W′ (pronounced “W-prime”) is the finite amount of work you can deliver above your Critical Power before you blow up. Think of it as your attack battery: every attack, every surge, every gap you close draws energy out of that battery. Once it's completely empty, that's the end of what you can still put out. (Jones & Vanhatalo, 2017)
Say your CP is 270 watts and your W′ is 15 kJ — that is 15,000 joules in your battery. Those are the two numbers in the chart above.
You ride 350 watts. That is 80 watts above your CP, so 80 joules drain from your battery every second. 15,000 ÷ 80 = 188 seconds: just over three minutes, and it is empty.
The same rider at 320 watts: 15,000 ÷ 50 = 300 seconds, five minutes. Thirty watts less, almost two minutes longer.
Those are exactly the two hatched areas in the chart: tall and narrow (350 watts, 3 minutes) or lower and wide (320 watts, 5 minutes). The same size — 15 kJ each, because it is the same battery.
time = W′ ÷ (power − CP)
It is a model, not a guarantee: it assumes your battery is full at the start and doesn't recharge along the way. But it does the sum with your two numbers instead of your FTP alone.
Two numbers, not one. CP is your endurance engine, W′ is your attack battery. Two riders with the same FTP can have a completely different W′ — and therefore have to race in fundamentally different ways. And just as important: their hard training blocks belong at different wattages. A single number, FTP, simply doesn't show that.
W′ recovery and W′ balance
The part most apps skip: as soon as you drop below your CP, your battery starts recharging. Three terms tend to get mixed up around that recharge, and keeping them apart saves a lot of confusion.
W′ — the size of your battery: the total work you can deliver above your CP.
W′ recovery — what comes back during recovery or rest. Ride below your CP for a while and part of it returns. In the literature: W′rec.
W′ balance — the energy live in your battery right now. In the literature: W′bal.
How fast that recharge happens varies strongly from rider to rider. Recovery goes fast at first and then slowly — in science that time factor is called tau: a fast phase (seconds) and a slow phase (minutes). The vast majority of training apps don't use CP, W′ or W′ recovery at all.
What you can do with it
Once you know your CP and W′, you know two things FTP alone won't tell you: what your endurance engine delivers, and how much you have above it. With that you set the duration and intensity of intervals you can handle exactly; FTP alone cannot do that. It also gives you a handle on the race itself: when to attack and when to save.
How fast you recover — the third number
One thing is still missing, and in a race it is often the decisive one: how fast your battery recharges. Whoever is full again first after every surge can take the next one. That pace varies strongly from rider to rider and doesn't come out of a formula. The research points to your aerobic engine — VO₂max and CP — as the keys to improving your recovery. (Chorley et al., 2020) A separate piece on that is coming.
Curious about your own profile?
The CP test maps your Critical Power and W′, with my analysis by email — the first step towards your personal recovery profile.