The range figure on a spec sheet is the output of a test, and the test was designed by the company selling the bike. Nobody is lying, exactly. A light rider on flat pavement in mild weather, in the lowest assist setting, with tires at full pressure and no cargo, really can travel that far. The problem is that almost nobody rides that way, and the conditions that produce the headline number are the conditions that almost never occur on a commute with a hill in it and two bags of groceries on the rack.
Watt-hours travel between bikes, advertised miles do not
Every battery carries a capacity in watt-hours, and if the sticker only shows volts and amp-hours, multiply them: 36 volts times 14 amp-hours gives 504 watt-hours. That single figure is the one thing you can carry from one showroom to the next without translation, because it describes stored energy rather than a marketing scenario. Range is watt-hours divided by the energy the bike actually spends per mile. Two bikes with the same capacity can differ by half in real distance, depending on drivetrain, tires, rider and terrain. Ask for capacity first, then treat the mileage claim as a hint.
What actually moves the energy-per-mile number
Consumption rises with everything that adds resistance or asks the motor to work harder. Total system weight matters most on climbs, because lifting a heavier rider plus a child seat plus a week of groceries up a two-hundred-foot grade costs energy that no amount of coasting returns. Assist level is the other big lever, and the jump from the lowest setting to the highest can easily double draw, since the motor is simply told to contribute more torque for the same pedal effort. Cold weather reduces usable capacity temporarily. Headwind, knobby tires, soft pressure and frequent stops all take their cut too.
Comparing two candidates without guessing
Put the bikes side by side on paper before you put them side by side on the road. A 400 watt-hour pack on a light commuter with efficient tires and a rider who prefers modest assist may outlast a 700 watt-hour pack on a heavy cargo bike hauling forty pounds up a ridge, and the smaller number wins that matchup honestly. What a careful reader checks is the pairing: capacity against the weight the bike is meant to carry, and the terrain it will see most days. Then check whether the display shows percentage only, or actual watt-hours and consumption, because the second kind lets you measure your own numbers within a week.
Charging habits, and what they cost over years
Lithium packs age from time, heat and the amount of time they spend sitting at the extremes of charge. Topping up to full the night before a long ride is fine; leaving the bike plugged in at one hundred percent for weeks in a hot garage is the habit that quietly shortens service life. Charging in the eighties and discharging into the twenties covers most daily riding comfortably, and storing a bike over winter at roughly half charge in a cool indoor spot is the standard advice from manufacturers. The Department of Energy oversees research on lithium-ion battery performance and durability, and the guidance that reaches consumers is consistent on avoiding heat.
Replacement packs and the case for a spare
A replacement battery is the largest single expense in owning an electric bike, typically several hundred dollars and sometimes far more on proprietary integrated systems, so ask the dealer for the current price of the exact pack before you buy the bike, not after. Availability matters as much as price: a pack that only one importer stocks is a risk five years out. A spare is worth carrying when your route is long, cold, hilly and loaded, and worth skipping when you can charge at work, because a second pack costs real money, adds several pounds, and ages on the shelf whether you use it or not.
The useful move is to log your own consumption for two weeks, with your normal cargo, your normal assist setting and your normal route, and then divide your battery capacity by what you measured. That number is yours, it will not appear in any brochure, and it is the one that tells you whether the hill and the shopping still leave you a comfortable margin home.