Bulb Efficiency at less than watt rating

[Pacecar]

For a halogen bulb spotlight using a cigarette lighter connection to an external 12Vbattery, and equipped with a Lightforce BPS electronic dimmer (high efficiency), and only needing about 200,000 CP for wildlife scanning .... now my question - for the same light output, would the battery consumption be less if I used a lower wattage bulb (e.g. 50W or 75W rating) or keep the 100W bulb with the dimmer tuning the light way down? Is the bulb efficiency (light production vs. electrical juice required) poorer when the bulb is operated at less than its rated wattage?

 

[IMSabbel]

Efficiency of incan bulbs has an extreme dependency on how hard you drive them.
You will be better of with a bulb rated for the lower power than underdriving the existing one.
(if they are of the same class)

 

[Bullzeyebill]

Better efficiency is achieved when higher voltage is applied. See Lux's destructive bulb testing sticky in the battery, electronics forum.

Bill