05 / Estimated power
Power, without a power meter.
Bunch estimates your power for every second of moving time from your speed, the road grade, how fast you’re speeding up, and the weight of you and your bike. It needs your weight, from Apple Health or entered by hand.
P = ( m·g·v·(grade + Crr) + ½·ρ·CdA·v³ + m·a·v ) / η- m
- Your weight + bike weight (bike defaults to 9 kg)
- g
- 9.81 m/s²
- Crr
- 0.005, rolling resistance for road tires on asphalt
- CdA
- 0.32 m², drag area riding on the hoods
- ρ
- 1.225 kg/m³ at sea level, thinning with altitude
- η
- 0.97, drivetrain efficiency
The constants
Second by second
- Speed
- GPS speed averaged over the last 3 seconds.
- Grade
- From the smoothed elevation profile over about 200 m, capped at ±20%, and limited so you can’t climb faster than 0.6 m/s (anything faster is altitude noise).
- Acceleration
- Speed change over 10 seconds, capped at ±1 m/s².
- Coasting
- Below 1.0 m/s or freewheeling counts as 0 W. Estimates are capped at 2,000 W.
On the ride page
- Average power
- The mean over moving seconds, with coasting counted as zero, like a power meter.
- Normalized power
- A 30-second rolling average, each value raised to the 4th power, averaged, then the 4th root. Only on rides with 10+ minutes of moving time.
- Max
- Your best 5-second average.
- Best efforts
- Best average over 5 s, 1 min, 5 min and 20 min, with W/kg.
- Energy & calories
- Energy is the sum of watts in kJ. Calories ≈ kJ ÷ (0.24 × 4.184), assuming your muscles are about 24% efficient, so roughly 1 kcal per kJ.
How accurate is it? About ±10% on climbs. On the flat it’s rough, because wind isn’t counted, and it reads high when you’re drafting. That’s why estimated power is left out of group-ride comparisons except on climbs.