Hub motors are broken up into 2 types...
1.) Small Geared
2.) Larger Direct Drive
Small geared are usually in the 250-350w range.. They are lighter, more torquey, but have less top end.
Direct drive are usually in the 500-1000w range.. They are heavier (not by much) and are capable of much faster speeds...
To speak to the "wattage" of a motor is a misnomer...
The wattage is USALLY set by the controller that runs the motor..
(i.e. This is no difference between a 500w and a 1000w "motor", it's the controller that provides more power (amps) and is the controlling factor..)
Most geared motors will handle up to 500w, after that, you really need a direct drive..
DD motors can usually handle 1200w continuous, without issue..
The more watts you start pushing into the motors, the more that the energy put in is transfered to heat.
As a general rule...
24v will get you 15mph top speed (usually 250-350w)
36v will get you 20mph (350-750w)
48v will get you 25mph (750w and up)
The equation is V olts times A mps = W atts or VxA=W
As you will notice, all of those figures equate to 10-15 amps on average...
(24v X 10a = 240w, 24v X 15a = 360w)
(36x X 10a = 360w, 36v X 15a = 540w)
Etc, etc...
So a 36v 15a controller will provide up to 540w (36v X 15a)
a.k.a. A 36v 500w motor
If the controller was a 30a one, then I would be (36v X 30a) = 960w
Or a 36v 1000w motor.
Hope that all makes sense.