How many watts does this use?

The transformer has following data - 220V 18A

~4000W Is the light very bright?

The light is very bright. However, a comparable modern lamp is brighter.

What is the diameter of the carbon rods used in this lamp?

To measure exactly, I must again on my attic. I estimate this of the diameters on 10mm.

Is this the same kind of technology that was used as streetlighting long time ago. I read something about "arc lamps" are used in pubilc lighting.

Yes - this is the same technology as in the old streetlamps. With the streetlamps, there were only other carbon rods. And a better protection against the weather.