I tend to keep late hours, and this extends my use of lighting. My main Lounge/dining area has 15 40 watt bulbs. Or it did. Now I have 15 LEDs. They cost £6 each and have a claimed life expectancy over 20000 hours. This means they come out at about 30p per thousand hours to REPLACE, compared with say 50p each for incandescent bulbs of the old type, lasting typically 1000 hours. So the ongoing replacement cost of the LED is about half the old type, per thousand hours use. Not significant compared with energy savings.
Now, using sunset time data through the year, assuming lights in my main room go on at 1 hour before sunset and off at midnight, I find that each 4w LED, replacing a 40w bulb, will burn for 2288 hours per year, saving £9.89 in electricity. As I have 15 such bulbs in my main room, so the saving is about £148 per year. That is just that room, just the evening, and ignores any use in the morning or on dull winter days.
If the life is the 20000 hours it says on the box, they should last about nine years. In fact my normal hours usually go past midnight, so with lights out at 1AM, the saving per year rises to £11.49 each, annually £172.32, with a burn time of 2659 hours. On these figures, the big investment in LED lamps made all in one go will be repaid well within the year.
I have assumed a unit price of 12p/kWh.
I have long suspected that I used more energy on light than on appliances (and I heat by gas). I now only have a few tiny halogens that are on for only short times, so would not pay back the cost of replacement in a reasonable time; All the lights that are on for hours at a time are now LED, the rest (until they wear out) compact fluorescent. My electricity bill is certainly showing an improvement and the year is far from up yet.