E-bike hill climb watts.

Worked example

A 100 kg rider and bike on an 8% grade at 15 km/h (4.17 m/s), Crr 0.005, CdA 0.6 m², 95% drivetrain, no wind. Climbing: 4.17 × 100 × 9.8 × sin(atan 0.08) = 326 W. Rolling: 20 W. Air drag: 27 W. That is 373 W at the wheel. You pedal 100 W, of which 95 W reaches the wheel, so the motor must supply 373 − 95 = 278 W. On the flat at 15 km/h the same ride needs only 47 W at the wheel. At the same speed and 100 W of pedalling, 3%, 5%, 10% and 12% grades need motor watts of 74, 156, 358, 438.

How it works

Wheel watts = climbing + rolling resistance + air drag at steady speed. Climbing power is speed × mass × g × sin(grade angle), the rate you gain potential energy. Rolling resistance is speed × mass × g × cos(grade angle) × Crr. Air drag is ½ × 1.225 × (air speed)² × speed × CdA. Motor watts = wheel watts − your pedal watts × drivetrain efficiency, never below zero.

Climbing power grows in direct proportion to both speed and mass, so a rider and load 10 percent heavier need about 10 percent more climbing watts at the same speed. Air drag grows with the cube of speed, so it matters little on a slow climb and most at speed on the flat.

Limits. Motor and battery efficiency are not modelled, so the battery supplies more than the motor watts shown. Rolling resistance is held constant. Many motors are rated by nominal watts, and peak torque or local rules may cap real output.

Sources and basis (opened 2026-10-08): Wikipedia, Bicycle performance, Power required section (the air drag, rolling resistance and climbing power formulas, total power divided by drivetrain efficiency; air density 1.225 kg/m³; low-speed rolling resistance coefficients of 0.003 to 0.006 for a variety of tires, rising about 50% at 10 m/s) and Mechanical efficiency section (drivetrain transmission 1–7% lower with derailleur gearing).

FAQ

How many watts does it take to climb an 8% hill on a bike?

For a 100 kg rider and bike at 15 km/h, about 373 W at the wheel with these defaults, 326 W of it just lifting the mass.

What size motor do I need for hills?

It depends on weight, grade, speed and how much you pedal. At 8% and 15 km/h with 100 W of your own, the motor must supply about 278 W at the wheel. Steeper grades or heavier loads need more.

Why does weight matter so much uphill?

Climbing power is proportional to mass. Air drag does not depend on mass, but on a steep climb it is a small share of the total.

Does a faster climb need much more power?

Climbing and rolling power rise in direct proportion to speed, so doubling speed roughly doubles those two. Air drag rises faster, with the cube of speed.

Is this the electrical power from the battery?

No. It is mechanical power at the wheel. Motor and battery losses make the electrical draw higher, so use measured battery draw if you need Wh per km.