For example we converted 10 50W halogen spots in the kitchen/dining area to LEDs.
Before LEDs: 500W total
After LEDs: 50W total
After LEDs: 0.9p/hour to run.
Over the 3 months of winter, figuring on 8 hours use per day, that's £64.80 in electricity for the old ones. But only £6.48 in electricity for the new ones.
But of course it's greener too.