Efficiency Factor measures how many watts you produce per heartbeat. It's one of the best ways to track aerobic fitness development over time.
The Formula
Efficiency = Weighted Power / Average Heart Rate
WattPlan uses Weighted Power when available (more accurate for variable rides) and falls back to average power.
Example: 210w Weighted Power at 140 bpm = an Efficiency of 1.50
Reading Your EF
A higher number means better cardiovascular efficiency:
- Rising EF at the same power = Your heart is working less hard (fitness improving)
- Rising EF at higher power = You're producing more watts at similar effort (getting faster)
- Falling EF = Possible fatigue, illness, or detraining
EF by Zone
WattPlan tracks EF separately for each training zone:
| Zone | Intensity Range | What Efficiency Tells You |
|---|---|---|
| Endurance | < 0.75 | Aerobic base efficiency |
| Sweet Spot | 0.75-0.90 | Sustained power efficiency |
| Threshold | 0.90-1.05 | Race-pace efficiency |
| VO2max | > 1.05 | High-intensity efficiency |
Compare EF within the same zone over time. Mixing zones makes comparison unreliable.
Where to Find It
Metrics Explorer (Stats Page)
Select "Efficiency Factor" from the metrics dropdown to see your EF plotted over time with weekly averages.
Ride Summary
After each ride, EF is calculated and stored automatically. Look for it in your activity details.
AI Coach
When WattPlan detects a significant EF improvement (5%+ over 4+ rides in a zone), you'll receive a notification. Tap it to discuss what the improvement means with your AI coach.
Tips for Reliable EF Data
- Use a chest strap HR monitor - Optical sensors can lag and produce noisy data
- Compare similar ride types - Endurance-to-endurance, not endurance-to-intervals
- Allow proper warm-up - First 10-15 minutes can skew HR data
- Look at 4-8 week trends - Single rides are noisy; trends reveal the truth
- Indoor rides are most consistent - No wind, terrain, or temperature variation
EF and Other Metrics
| Metric | Relationship to EF |
|---|---|
| Fitness | Training volume; EF shows how your body responds to that volume |
| FTP | Maximum sustainable power; EF shows the cardiovascular cost of producing it |
| Decoupling | How well you maintain EF throughout a ride |
| HR Zones | EF combines power and HR into a single efficiency number |