What the heck is POWER anyway?
Generally speaking, power is the force that makes your electric vehicle go. The force at the wheel is transferred from the force from the motor , which is supplied by batteries through a controller module.
Power is also the electric force that makes the electric motor turn. It is the product of the volts and the amps delivered by the batteries and controller. Gasoline motors usually are described or "rated" in horsepower, but small EV motors are usually "rated" in watts: 746 watts = 1 horsepower.
When a motor is described as being "250W motor", it is rated to be able to deliver about 1/3hp continuously without stalling or overheating. Its rating will specify what voltage and amperage it needs to deliver the power at a specific rotational speed, usually described as revolutions per minute ( RPM). Of course, you cannot expect that 250W motor to continuously handle a task or "load" that requires more than 250W of continuous power; any more than you would expect a single-engine to power a multi-engine airplane for very long, if at all.
So, when you consider power for an EV, you will need to consider how much power at the wheel will be required to go the speed you want: on flat roads, up hills and with how much cargo. It all takes power. Faster, higher and heavier all take more power (see chart in post below). When you determine your requirements for power at the wheel, you can consider the power a system can deliver.
There is no 100% efficient system:
all systems have components with inherent losses that diminish the output of power. Add them all up, and you may find total system efficiency and total power output is much lower than you expected. If you need 500W at the wheel, the losses from wind resistance, tire resistance, motor heat, controller losses , total weight and battery voltage-sag may require you to use a 750W motor, bigger controller and more batteries.
There are trade-offs all along the line: you may want higher voltage, but lower amps... or the opposite. You may want a heavier bike, but lighter batteries. Each component you consider will have advantages AND limitations... The good news is: there is plenty of power available for just about any setup you might want. What you choose for your setup depends on your goals, your efforts and your finances.