Originally Posted by Merriwether
Notice that the Metal Halide lamps (HID) are rated at more than double the lumens per watt of halogen lamps. Now, there's no separate entry for "overvolted" halogen lamps, but overvolting halogen bulbs doesn't increase lumen-watt efficiency by more than about 20% or so. It certainly doesn't increase efficiency by more than 100%!
when you overvold a halogen it gets very effient. a 20 watt 12v bulb at 16v will put out just about 40 watts of light. but it uses less power then a 40 watt halogen bulb. though the life of the bulb is reduced to about 20 to 30 hours. add more power and you get better results. but a bigger benifit is you get a whiter light too. though this formula works with HID lights it's not real practical to do it. most of the halogen systems are overvolted a ltitle bit. but it takes a good controller to do it right. and I don't think companies would get too far if they told people about the bulb life and charged then the usual 20.00 for a replacement bulb. where you can get the same bulb for around 3.00 if you know where to look.