There is a misconception that wattage is a unit of brightness. Wattage is a unit of power usage (as seen in higher electric bills). Brightness is actually measured in lumens.
The true wattage rating (the actual power drawn) is the lower rating listed on the light bulb package. The higher wattage rating is supposedly the equivalent wattage for a comparable incandescent bulb.
To get the best brightness for the lowest wattage, divide the lumens by the true wattage. The higher the number, the more efficient the bulb is.
The highest true wattage found is 13 watts (100 watt equivalent, 900 lumens).
Due to wear and tear, the brightness of an LED bulb can diminish over time.