All lamps or light bulbs have letters and numbers stamped on them which indicate the power consumed by bulb operation at the nominal operating voltage.
For instance, in a 12V / 21W bulb the filament will consume 21 watts of power when 12 volts is applied across the filament.
While the wattage is not necessarily an indication of light output, it can be generally assumed that, for similar types of lamps, the higher the wattage the greater the light output will be.
