The house I bought has halogen downlights pretty much everywhere, and they are all 12 V. I was wondering why bother with stepping down to 12 V but didn't think much about it.
When looking at downlights in some shop the other day (thinking about replacing them with LEDs) I noticed that for halogen 12 V ones are much more efficient than 220 V ones - for 50 W specs were 350 lm for 220 V and 680 lm for 12V. How can there be such a difference for the same technology but different voltages?
You can check specs for the same manufacturer (Osram) and the 2 voltages here and here
This gets into efficacy/usefulness verses efficiency and the important fact that less voltage is far more stabler.
If you look closely:
12V 50W energy consumption = 53 kWh/1000h
240V 50W energy consumption = 50 kWh/1000h
Thus the 240V 50W is 3kWh/1000h more efficient in energy consumption.
However the 240V 50W has half the life span of the 12V
12V 50W number of switching cycles= 50000
240V 50W number of switching cycles = 25000
Thus the efficiency you would gain is nullified due to the decreased lifespan.
Next compare the luminous intensity
12V 50W luminous intensity = 1450
240V 50W luminous intensity = 900
By far the 12V has a much greater efficacy to efficiency ratio.
Now to explain why
Voltage is the rate/speed/pressure of electrical phenomenon.
On average 12 volts is plenty fast to illuminate a 50 Watt incandescent or halogen.
Increasing the voltage beyond this point as you can see does little help as the efficacy "usefulness" of the bulb to achieve desired illumination in relation to the efficient use of energy it requires is most satisfactory at 12 Volts.
As an experiment, take a 9 volt battery and touch the two prongs to a 20 watt halogen MR16 or MR11. The voltage required to reach the desired efficacy is very little, and less voltage/pressure is a good thing.