How Electric Wheelchair Range Is Actually Calculated (Wh, Weight and the 40% Rule)

Every wheelchair spec sheet prints a range number. Almost none explain how it was measured — which is why buyers are angry by week two. Here is what actually happens inside that number, and how to read it honestly.

The watt-hour budget

Range starts with battery energy in watt-hours: voltage × amp-hours. A 24V 12Ah pack stores 288Wh. A chair cruising on flat pavement at 4–6 km/h draws roughly 150–250W depending on weight and surface. Division says 288Wh ÷ 200W ≈ 1.4 hours ≈ 7–8 km of gentle driving. If a spec sheet claims 25 km on 288Wh, the fine print assumed a 50 kg rider, a dead-flat test track, and brand-new cells at 25°C.

Rider weight is the biggest variable

Rolling resistance and drivetrain load scale with total mass. Going from a 70 kg rider to a 100 kg rider typically cuts range 15–25%, and add cargo on top and it falls further. Our bench data across the 6000 series: the same 20Ah pack delivered 24 km with a 65 kg test rider, 19 km at 85 kg, 15.5 km at 105 kg on the same loop.

What the BMS takes back

The battery management system reserves the bottom 8–12% of capacity to prevent cell damage, and another margin at the top for charging safety. Usable energy is therefore about 85–90% of the nameplate Wh. Motors and controllers also lose 10–15% as heat under load. Neither loss appears in marketing range.

Temperature and terrain

Lithium capacity drops roughly 15–20% at −5°C and recovers when warm. Lead-acid sags similarly in cold and recovers more slowly. Carpet adds 20–40% draw versus pavement; a 5° ramp can double instantaneous consumption; stop-start urban driving costs 10–15% versus steady cruising.

The honest conversion

Take the rated range and multiply by 0.6–0.7 for a realistic daily expectation for a typical adult rider on mixed terrain. If that number still covers your day, the chair fits. If it exactly matches your day, buy the bigger battery — the 20Ah option exists precisely because the 12Ah "rated range" looked sufficient on paper and wasn't.