You know your FTP. You train with power zones. But have you ever wondered:
- Why did I blow up on that climb when my power was "only" 20 watts above threshold?
- How long can I actually hold 350 watts before I crack?
- Why can some riders attack repeatedly while others fade after one effort?
The answer is W' (pronounced "W prime").
Understanding W' transformed how elite cyclists and coaches think about performance. Now it's available to everyone—including in WattPlan's workout player.
This guide explains everything you need to know, without requiring a sports science degree.
The Two-Tank Model
Your body has two energy systems for cycling:
Tank 1: The Diesel Engine (Critical Power)
This is your sustainable power—the effort you can maintain for a long time without blowing up. It's essentially your FTP, the boundary between "hard but manageable" and "I'm dying."
- Runs on aerobic metabolism (oxygen)
- Virtually unlimited duration
- What you use for time trials, long climbs, endurance rides
Tank 2: The Nitrous Boost (W')
This is your finite reserve for efforts above threshold. Think of it as a battery that:
- Drains when you ride above Critical Power
- Recharges when you ride below Critical Power
- Runs out completely = you crack
W' is measured in kilojoules (kJ)—typically 15-25 kJ for most cyclists.
A Practical Example
Let's say you have:
- Critical Power: 250 watts
- W': 20,000 Joules (20 kJ)
You attack at 350 watts (100W above CP). How long until you blow?
Time to exhaustion = W' ÷ (Power - CP)
= 20,000 ÷ (350 - 250)
= 20,000 ÷ 100
= 200 seconds
= 3 minutes 20 secondsAfter 3:20 at 350W, your W' tank is empty. You either slow down or you crack.
But here's the key insight: if you drop below CP, your W' starts refilling.
That's why smart racers surge, recover, surge, recover—they're managing their W' tank.
W'bal: Your Real-Time Fuel Gauge
W'bal (W' balance) is how much of your W' tank remains at any moment.
In WattPlan's workout player, you'll see this as a gauge:
W'bal: ████████░░ 78%
TTE: 2:34 at current power- Above CP: The gauge drains
- Below CP: The gauge refills
- Near empty: You're about to crack
This lets you pace hard efforts intelligently instead of guessing.
How W' Differs from FTP
| Aspect | FTP | W' |
|---|---|---|
| What it measures | Threshold power | Capacity above threshold |
| Units | Watts | Kilojoules (kJ) |
| Duration | ~40-60 minutes | Seconds to minutes |
| Trainable? | Yes (aerobic) | Yes (anaerobic) |
| Used for | Zone setting, Load | Pacing hard efforts |
You need both numbers to understand your complete physiology.
A rider with high FTP but low W' is a "diesel"—great at time trials, struggles with attacks.
A rider with moderate FTP but high W' is "punchy"—can surge repeatedly, may fade on long efforts.
What Your W' Data Tells You After a Ride
After your workout, WattPlan shows:
W' Expended
Total anaerobic energy you used. Higher = harder ride (above threshold).
Lowest W'bal
How deep you dug into your reserves. Lower percentage = bigger effort.
- 50%+ = Moderate intensity
- 25-50% = Hard effort
- 10-25% = Very deep dig
- <10% = Near-maximal, close to cracking
Hard Efforts (Matches)
Visualizes your notable above-threshold efforts. Bigger flames = more energy burned in that effort.
Recovery Rate
How efficiently you recharged between hard efforts. Affected by:
- How far below CP you recovered
- Your individual physiology
- Fatigue level
Training to Improve W'
To Increase W' Capacity:
Short, hard intervals with full recovery:
- 30-second all-out efforts
- 1-2 minute max intervals
- Hill sprints
- Tabata-style work
To Improve W' Recovery:
Repeated efforts with incomplete recovery:
- 3×3 min at 110% FTP, 3 min recovery
- Over-under intervals
- Race simulations with surges
To Improve Critical Power:
Threshold and sweet spot work:
- 2×20 min at FTP
- Sweet spot intervals (88-94% FTP)
- Long tempo rides
Common Questions
Is Critical Power the same as FTP?
Almost. For 80% of athletes, they're within 5 watts. CP is slightly more scientifically robust, but for practical purposes, you can treat them as equivalent.
How do I find my W'?
WattPlan estimates it from your ride history and experience level. For a more precise measurement, you can do a CP test (3-minute + 12-minute all-out efforts).
Why does my recovery rate vary?
Recovery is highly individual. Elite athletes often recover faster than the standard model predicts. Fatigue, hydration, and how far below CP you drop all affect recovery speed.
Is the W'bal model perfectly accurate?
No model is perfect. Research shows W'bal predicts exhaustion well (91% accuracy) but recovery rates vary by individual. Use it as a guide, not gospel.
The Science (For the Curious)
The Critical Power model was first proposed by Monod and Scherrer in 1965. They observed that the relationship between power output and time to exhaustion follows a predictable curve.
Dr. Philip Skiba developed the W'bal algorithm in 2012, allowing real-time tracking of W' depletion and reconstitution during intermittent exercise.
Key research:
- Monod H & Scherrer J (1965). "The work capacity of a synergic muscular group." Ergonomics
- Skiba PF et al. (2012). "Modeling the expenditure and reconstitution of work capacity above critical power." Medicine & Science in Sports & Exercise
- Skiba PF et al. (2014). "Validation of a novel intermittent W' model for cycling using field data." International Journal of Sports Physiology and Performance
Start Using W' Today
In WattPlan:
- During workouts: Watch your W'bal gauge to pace hard efforts
- After rides: Check the Advanced tab for W' analysis
- Over time: Track your W' trends in Stats to see anaerobic fitness improve
Understanding W' won't make you faster by itself—but it will help you use the fitness you have more intelligently.
That could be the difference between bridging to the break or getting dropped.
Questions about W'? Contact us or check our Help Center for more details.