Ebike battery - volts and wattage Will a 36V 500W ebike with a 36V 17.5Ah battery go farther and faster than a 36V 250W ebike with a 36V 10Ah battery, or will I give up something by doubling that wattage?
The 500W bike will be able to go faster than the 250W one (unless the bikes assistance is speed limited). Note that it will not go twice as fast as the power required to overcome aerodynamic drag is proportional to velocity to the third power.

Higher power motor and battery combinations are not intrinsically  less efficient than lower power ones, so the bike with the higher battery capacity would have greater range, if both bike were ridden on the same route at equal speed (and have the same mass, drag coefficient, frontal area etc.)