[Madhatter]

There is a danger where you pass from lit to dark or dark to lit, can't remeber which way round it is.
Neil is right as usual, sodium vapour lights are the most efficient light you can get, down side being that they're orange, but that doesn't matter for roads, highlighting some buildings, and for some security purposes. Even if you used flourescents, which do get used in some signlights, they take time to warm up, so would be no good for roads.
Not that good for cameras though, but if you use a hallogen spot on a pir and shine it where its needed, down, not across, then it's not expensive to run, and not light polluting. I do agree with niel, who's right again, that a lot of the time a 150 would be plenty, especially just to light a path or small yard.
If you want constant cheap illumination for a camera, i'd use flourescents floods, although they can cause flicker on the camera, or mercury vapour , which is white like halogen.