Nominal Watts, Peak Watts and What the Controller Decides

Both approaches to motor power ratings work. What differs is what they cost you, and the cost is what this sets out.

Two numbers measuring different things

A nominal or continuous rating describes the power a motor can sustain indefinitely without overheating, and it is the figure most regulations refer to. A peak rating describes what the system can produce briefly, which is usually several times the continuous figure and limited by heat.

Marketing tends to quote whichever number is larger, and the two are frequently mixed within a single specification sheet. That is why two bikes with the same headline wattage can accelerate completely differently from a standstill. Ask specifically which figure is being quoted, and treat a specification that does not say as uninformative.

Torque is what you feel at a standstill

Torque is rotational force, and it is what gets a loaded bike moving from stationary or holds speed on a gradient. Power is torque multiplied by rotational speed, so a motor can have high peak power and still feel weak at very low speeds.

This is why torque figures in newton metres are often more informative than watts for anyone carrying loads or climbing hills. A mid-drive multiplies its torque through the bike's gears, so its useful torque at the wheel depends on which gear you selected. A hub motor's torque at the wheel is whatever the motor produces, which is why hub motors need more of it to climb the same hill.

What the controller is doing

The controller sits between battery and motor and decides how much current to deliver at each moment, based on sensors, assist level and its own limits. Its current limit, more than the motor's rating, determines how sharply the bike accelerates and how it behaves under sustained load.

Controllers also apply thermal protection, reducing power when motor or controller temperature rises to protect the components. That protection is why a bike can feel strong at the bottom of a long climb and noticeably weaker near the top. A well-tuned controller with a modest motor usually produces a nicer bike than a powerful motor with crude control.

Why bigger is not automatically better

A higher-powered system draws more current, which empties the battery faster and generates more heat throughout the drivetrain. It also puts more stress on the chain, sprockets and spokes, which on a converted bicycle may not be built for it. Legal classifications in most countries set limits on continuous power and assisted speed, and exceeding them can change the vehicle's legal status entirely.

Come the wet months, that may bring requirements for registration, insurance, licensing or helmets, and the rules differ substantially between countries and sometimes between regions.

Check what applies where you ride before buying anything advertised as high powered, because the consequences are legal rather than mechanical.

Reading a specification sheet critically

Look for continuous power, peak power, torque, battery watt-hours and controller current, and note how many of them are absent. A seller quoting only peak watts and amp-hours has given you two of the least informative numbers available. Efficiency figures are rarely quoted and vary with load, so they are hard to use for comparison even when present.

Reviews that measure hill-climbing performance with a stated rider weight are far more useful than any manufacturer figure. If you cannot get straight answers about ratings, that is information about the seller rather than a gap in your knowledge.

Matching the system to the job

A flat commute with light luggage needs very little, and a modest system will be quieter, lighter and gentler on the battery. Hills, cargo, a trailer or a passenger all argue for more torque rather than simply more peak power.

With a loaded rack, long distances argue for battery capacity and efficiency rather than power, since a strong motor with a small pack is a short ride. If you regularly carry a heavy load, look for a system rated for that use rather than one that merely manages it. The best system for most commuters is the smallest one that comfortably handles the worst hill on their route.

Side by side

The takeaway

Read the continuous rating and the torque figure, and treat the peak number as advertising.

Fix the small rattle. Every large repair began as one.