Little known fact, popular as they are with interior designers, halogen bulbs are in fact one of the least efficient of all light sources. Only incandescent bulbs are less efficient. there is probably a perception gap here, halogens are frequently marketed as "Low Voltage", which many less technical consumers will interpret as "Low Power".

Lets put some metrics on the issue, Incandescent bulbs turn out 17 Lumens per watt, halogens however around 20 Lumens per Watt whilst standard compact fluorescent bulbs start around 48 l/watt. Highly efficient fluorescent efficiencies are currently in the region of 90 l/watt.

A global switch to efficient lighting systems would trim the world's electricity bill by nearly one-tenth the International Energy Agency (IEA) says in a report, which it claims is the first global survey of lighting uses and costs. 19% of the worlds energy consumption is given over to lighting. Switching to energy efficient systems would slice this in half (Leading to a 10% saving) 

 To put this in context, in the UK and assuming we made no other changes, we are looking at the potential to mothball entire fossil fuelled power stations.

The carbon dioxide emissions saved by such a switch would, the report concludes, dwarf cuts so far achieved by adopting wind, solar power & other renewables. the report also notes that although incandescent bulbs are the biggest efficiency offender, fluorescent tubes are the biggest consumer as they are extensively deployed in business premises who are basically just bad at turning them off. There is also a perception that fluorescent tubes are highly efficient however the reality is that the efficiency of fluorescents varies between 15% & 60%.