A 40 watt halogen lamp can be replaced by a 4 watt LED. This can pay for itself in less than a year in some instances. eg
One 40 watt halogen on 4 hours a day will cost approx €11.68 to run over a year.
One 4 Watt Led on 4 hours a day will cost approx €1.17 to run over a year.
LED lamps also last a lot longer and therefor will save a lot more.