WHO THIS IS FOR
IS THIS YOU?
The data-driven rider who's heard of critical power
You track FTP and TSS but keep seeing 'CP' and 'W prime' and want to understand what they actually mean.
The racer who keeps emptying the tank too early
You burn through your hard efforts in the first half of a race and have nothing left for the decisive moment.
THE ROADMAN VIEW
The Roadman view
Alex Welburn brought this to the podcast in a way that finally made it land. The metrics most amateurs live by — FTP, TSS, CTL — describe your sustainable engine. They tell you nothing about the bit that actually wins races: the finite reserve you can spend above that sustainable ceiling. That reserve has a name, W', and unlike a lot of sports-science jargon, it's a concrete, measurable number you can train.
The model behind it is the critical power concept. Your power-duration curve has two parts: critical power, the asymptote you can theoretically hold for a long time, and W', the fixed pool of work you can do above it. Welburn's framing is the rechargeable battery — every attack, every surge, every effort over critical power draws the battery down, and it only recharges when you drop back below. Empty the battery and you're done, no matter how good your FTP looks on paper.
Here's where it gets genuinely useful. W'bal — the running calculation of how much battery you have left — turns this from theory into a race tool. Pacing a hard climb or a breakaway becomes a question of managing the battery, not just holding a power number. And the battery itself is trainable: repeated short maximal efforts enlarge it. We've covered the session side of this in the anaerobic capacity answer — this one is about understanding the number so you can actually use it.
EXPERT EVIDENCE
WHAT THE EXPERTS SAY
- Alex WelburnCycling coach and physiologist; PhD researcher on critical power and W'
W' is the finite quantity of work, in kilojoules, that a rider can perform above critical power — and it functions like a battery that drains during hard efforts and recharges below threshold. It's measurable, trainable, and far more predictive of what happens in the decisive moments of a race than FTP or training-load metrics alone.
Hear it: Why Your CTL Is Wrong | Roadman Cycling Podcast - Dr Tim PodlogarNutrition consultant, Tudor Pro Cycling; researcher, University of Birmingham
The capacity to repeatedly perform high-intensity efforts depends on muscle glycogen availability as well as the size of the anaerobic reserve. A rider who arrives at a race under-fuelled cannot fully access or refill their W' — the battery is there, but the fuel to recharge it isn't.
Hear it: Race Weight & Carb Timing Mistakes | Roadman Cycling Podcast
PRACTICAL APPLICATION
DO THIS WEEK
Estimate your critical power and W'
Do two maximal efforts on separate days — typically a 3-minute and a 12-minute all-out test. Most modern platforms (WKO, Intervals.icu, TrainingPeaks) compute critical power and W' from the data. The W' figure, in kilojoules, is your anaerobic battery size.
Train the battery bigger with short maximal repeats
Sessions of 8–12 efforts of 30–60 seconds well above critical power, with enough recovery to go hard again, enlarge W' over a 4–6 week block. This is the same family of work as anaerobic capacity training — short, maximal, repeated.
Use W'bal to pace races
If your head unit or analysis platform supports W'bal, watch it during hard efforts: it shows the battery draining and recharging live. Don't fully empty it before the decisive moment. Spending early surges you can't afford is how riders blow up with kilometres to go.
COMMON MISTAKES
WHAT CYCLISTS GET WRONG
MISTAKEConfusing W' with FTP or critical power.
FIXFTP and critical power describe your sustainable ceiling. W' is the separate, finite reserve above that ceiling. They're different numbers measuring different things — you need both to model your real capability.
MISTAKEEmptying your W' battery early in a race.
FIXEvery surge above critical power drains the battery. Spend it deliberately and keep enough for the decisive effort. W'bal tracking makes this visible — without it, riders routinely run empty before the finish.
MISTAKEAssuming W' is fixed and untrainable.
FIXW' enlarges with repeated short maximal efforts over a focused block. It's one of the more trainable physiological qualities — the battery genuinely gets bigger with the right work.
FAQ
FREQUENTLY ASKED QUESTIONS
What does W' stand for in cycling?
What is a good W' value for a cyclist?
How is W' different from critical power?
What is W'bal?
Can you train W' to be bigger?
Do I need a special device to measure W'?
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