The wattage (power consumption) of an LED can be easily calculated using Ohms Law.
P = V x I
P = power in watts consumed by the LED
V = the forward voltage rating of the LED (Vf)
I = the current used to drive LED
For example, the forward voltage of an LXML-PWC2 LED is 2.9 Vf. If you are powering the LED at 700mA (0.7A), then the power consumption for the LED will be 2.1W.
2.9 x 0.7 = 2.03W
Note that unlike traditional incandescent lighting, which typically equates light output to wattage; the wattage of an LED is only an expression of its power consumption, not its light output. Light output for LEDs is always expressed in Lumens. Not watts.