Watt-hours, not miles: how to work out the real range of an electric bike
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The range figure in an electric bike listing is not a measurement of your commute. It is a measurement of a test the manufacturer designed, and you are almost never told the conditions. The good news is that the number you actually need is printed on the same page, and you can compute a realistic range from it in about thirty seconds.
The one number that matters: watt-hours
A battery's capacity is measured in watt-hours (Wh). It is the product of two figures that listings almost always publish:
volts (V) × amp-hours (Ah) = watt-hours (Wh) 48 V × 15 Ah = 720 Wh 36 V × 10.4 Ah = 374 Wh 52 V × 20 Ah = 1,040 Wh
Watt-hours are how much energy the pack stores. Volts and amp-hours on their own are not comparable across bikes: a 36 V 14 Ah pack (504 Wh) holds more energy than a 48 V 10 Ah pack (480 Wh), even though the second one has the bigger headline voltage. If a listing gives you volts and amp-hours but no watt-hour figure, multiply them yourself before comparing anything.
A listing that publishes neither amp-hours nor watt-hours — only “48V lithium battery” — has withheld the single most useful number about the product. That is a finding about the seller, not a gap for you to fill with an assumption.
What a mile costs
Energy consumption is measured in watt-hours per mile. For an electric bike carrying an adult rider, the realistic band is roughly:
| Riding | Typical consumption | What it looks like |
|---|---|---|
| Low assist, flat, light rider, pedalling hard | 8–12 Wh/mi | The conditions most advertised range figures are measured under |
| Moderate assist, mixed terrain, ordinary effort | 15–20 Wh/mi | A realistic commute for most riders |
| High assist or throttle, hills, heavier rider, cargo | 25–35 Wh/mi | Throttle-only riding on a fat-tyre bike lives here |
Divide capacity by consumption and you have your range:
720 Wh ÷ 18 Wh/mi = 40 miles (moderate assist) 720 Wh ÷ 30 Wh/mi = 24 miles (throttle, hills) 720 Wh ÷ 10 Wh/mi = 72 miles (the number in the advert)
All three of those are the same bike. This is why two listings can honestly advertise “up to 70 miles” and “up to 25 miles” for packs of identical size — one quoted the top of the band and one quoted the bottom.
Why the advertised figure is the way it is
Manufacturers are not usually lying. They are quoting a best case, and the conditions that produce a best case are consistent enough to list:
- Lowest assist level, with the rider pedalling meaningfully throughout.
- Flat ground, no headwind, mild temperature.
- A light rider — frequently around 155–165 lb, sometimes unstated.
- A new battery at full charge, run down to empty.
- Correctly inflated tyres, which most bikes in the wild do not have.
None of those is dishonest on its own. Together they describe a ride that few people take.
Four things that quietly reduce the number
Usable capacity is smaller than nominal capacity. The battery management system holds back a reserve at both the top and the bottom of the pack to protect cell life. You do not get every advertised watt-hour, and you should not want to.
Cold weather costs capacity. Lithium-ion cells deliver noticeably less energy below freezing, and the loss is temporary — capacity comes back when the pack warms up. A winter commuter should size the pack for January, not for June.
Cells age. Cycle life is usually quoted as the number of full charge cycles before the pack falls to 80% of its original capacity, commonly in the 500–800 range for cells used in this class of product. Charging to 100% and leaving it there, and running to empty regularly, both accelerate that. For daily use, charging to roughly 80–90% and topping up more often is easier on the pack than deep cycles.
Rider and cargo weight matter more than riders expect, particularly on hills, where the energy cost is close to proportional to total mass.
What to look for in a battery specification
- Watt-hours stated directly, or volts and amp-hours from which you can compute them
- The cell manufacturer named — a listing that names its cells is making a checkable claim; one that says “A-grade lithium cells” is not
- Whether the pack is removable, and whether it locks to the frame
- Charger output in amps, which sets charging time: 720 Wh at a 2 A / 48 V charger (about 96 W) is roughly a 7–8 hour charge, not an overnight convenience but close to it
- Certification of the battery and the electrical system to a recognised safety standard, stated with the standard number rather than as “certified”
- Warranty term on the battery specifically, which is often shorter than the frame warranty
Working out your own number
Take the route you will actually ride. Multiply its round-trip distance by 20 Wh per mile if it is hilly or you expect to use high assist, or by 15 if it is flat and you intend to pedal. Add 30% headroom for cold, wind, ageing, and the fact that arriving at 0% is not a plan. The result is the pack size to shop for.
12 mi round trip × 20 Wh/mi = 240 Wh 240 Wh × 1.3 headroom = 312 Wh minimum pack
A 374 Wh bike does that commute comfortably. A 250 Wh bike advertised at “up to 40 miles” does not, and no amount of range claim in the listing changes it.
Related: peak watts, continuous watts and torque covers the other half of the drivetrain, and explains why the controller sets a ceiling on both.