As compared to what other type of yard light ?

Metal Halide is more efficient as compared to Mercury Vapor. And a cleaner white too.

Is this a residential or commercial light? Where did you find it?

I'm curious because I'm interested in replacing a couple of old MV with something more efficient. I've considered LPS, which is the efficiency king, but it seems to be available only in commercial products at a stratospheric price. Besides, its CRI is awful.

MH looks like a good compromise with reasonably high efficacy and good CRI.

davidr: where have you found any LPS. They are uncommon. In a more than a decade of commercial work, I can;t remember seeing a single one in use. If you notice from the poorly laid out (due to limitations in copy and paste) a HPS light is close in efficiency to a LPS and the HPS lights are typically cheaper than MH. MV actually seems to be the best as far as all around ratings but I personally do not like the light color they emit and most people do not understand they continue to dim throughout their life span. I see so many that are simply so dim they do not illuminate much of anything but as long as that thing is on, people just won;t replace the lamps. I prefer MH or an incandescent light. There are some newer compact florescents that have a very good color temp out now that I would consider.

Lighting type Efficacy
(lumens/watt) Lifetime
(hours) Color rendition index (CRI) Color temperature
(K) Indoors/Outdoors
-------------------------
High-Intensity Discharge
Mercury vapor 25Â60 16,000Â24,000 50 (poor to fair) 3200Â7000 (warm to cold) Outdoors
Metal halide 70Â115 5000Â20,000 70 (fair) 3700 (cold) Indoors/outdoors
High-pressure sodium 50Â140 16,000Â24,000 25 (poor) 2100 (warm) Outdoors
Low-Pressure Sodium 60Â150 12,000Â18,000 -44 (very poor) Outdoors

Right, LPS are very tough to find. I've seen sources that put LPS efficacy at up to 185 lumens/Watt, HPS at 140 l/W. But the light quality for LPS is pretty grim, HPS not too great either.