The resident Sparky at worked has explained it to me as such.

There are 4 types of HID Installations.

1. Single HID Bulb.

This install is for when you have a desperate High & Low Beam Light Housings, single headlight or when you don't wont High Beam on a dual headlight.

2. Combo bulb. For Single & Dual Headlight Housings. Has HID Low beam and Xenon High Beam. Cheap and quick to changeover from Hi/Low Beam, but you don't get the benefit of HID for High Beam. But can be wired so both come on at once.

3. Solenoid Bulb.

This is when the bulb uses a rotation assembly inside to the bulb that rotates for High and Low. Is slow, but cheap and what most of the Ebay ones are. For single headlight and dual headlights.

4. Dual Bulb.

This is the best option for Single and Dual Headlight housings and has 2 separate HID bulbs in one and can be wired so that High & Low Beam comes on at once, but requires Twice the Ballasts. If not wiring so that High & Low Beams come on at once then is still slow to change. A setup where high & low beam come on at once will boil water at 100m.

Just remember if buying off Ebay etc the Ballasts come in different sizes and some of the cheaper ones have a Ballast the size of a housebrick. (Well almost )

Most of the cost of HID is with the bulbs themselves, so when buying a HID get ones that use Brand name Bulbs.

Finally HID's give off huge amounts of heat. Some plastic reflectors my be melted by this. But if you are lucky enough to have a glass reflector you can buy 50w HID's instead of the standard 35W ones. Good for Low Flying Aircraft etc.

 

Thanks Trips, always good to know - **** - boil water at 100M - must be like a friggin raygun....
I WANT one !