Some rules of thumb for tungsten filament bulbs such as halogen lamps:
Power is proportional to V^1.6 (if it were a constant resistance, it would be V^2, but the resistance of the filament goes up signficantly as it get hotter)
So at 18V it would draw about 96 watts.
The light output goes up as V^3.4, so at 18V the light output would be about 4 times the output at 12V.
The big drawback, though, is life, which is inversely proportional to V^16. More likely than not, 18V would heat the filament to the point of immediate failure, making the approximation invalid, but if it were, the life expectancy would be reduced by a factor of 650.
In the more normal range of small tweaks on voltage, some reasonable approximations is that 5% voltage change will be 20% change in light, and a doubling or halving of life. The power difference from 5% change in voltage is about 8%, rather than the 10% that would occur in a fixed value resistance.