Watt-Hour (Wh)

Why amp-hours mislead

Amp-hours only compare meaningfully at the same voltage. Watt-hours compare across everything, because they account for both:

Wh = nominal voltage × amp-hours

A 36V 15Ah pack is 540Wh. A 48V 15Ah pack is 720Wh. Identical amp-hour figures, one third more energy in the second. A buyer comparing “15Ah” against “15Ah” across two voltage platforms is comparing nothing at all.

This is the most common unit error in e-bike battery sourcing, and it works in both directions — a 52V 20Ah pack (1040Wh) beats a 48V 21Ah pack (1008Wh) despite the smaller amp-hour number.

Where the figure has teeth

Watt-hours are not just a comparison tool. They appear in regulation.

Transport rules classify lithium batteries partly by watt-hour rating, and the thresholds affect packing instructions, labelling, and what a carrier will accept — see shipping lithium batteries by air and sea. Several airlines and jurisdictions set watt-hour limits on batteries carried by passengers, which matters if your product is marketed as travel-friendly.

Some markets also use watt-hour or power thresholds in defining what counts as a legal e-bike rather than a moped, which changes the whole regulatory treatment of the vehicle.

Using it properly

Ask for Wh whenever you compare quotations, and check the supplier computed it from nominal voltage rather than from the fully charged voltage — using 54.6V instead of 48V inflates a 20Ah pack from 960Wh to 1092Wh, a 14% overstatement that is technically defensible and commercially misleading.

Resist converting watt-hours straight into a range claim. Consumption varies with rider weight, terrain, assist level, and temperature to a degree that makes a single km figure indefensible unless you publish the Wh/km assumption beside it.