Somonas is correct! Having a planted tank should not be made complicated however with so many terms floating around, it does get confusing.
The term 'Wattage' is most often used in the plant world because one can easily find out the wattage of the light by reading it off the light. Wattage is not the actual light output or intensity but wattage refers to how much power is being consumed by the light source (or bulb/tube/lamp). From a manufactures point of view, wattage is easily determined by measuring the power (voltage and current).
Lux on the other hand refers to the amount of light (illumination) present on a surface. Lux would be a much better way to go as you actually want to know how much light your plants are getting or require at a certain distance from the light. However, it is not practical because lux is environment dependant, meaning it depends on distance, the water, etc. therefore a manufactor cannot tell you the lux output unless he specifies from a certain distance such as 1000lux at 1' in a vaccum or whatever from the bulb/tube/lamp.
Another term that you may come across is Lumen. The term lumen refers to the quantity of light emitted by a lamp/tube/bulb or could also be called intensity.