if we assume 15p perkw = 41.25p an hour is £9.90 a day =£69 70p a week =£3600 a year
now you can possibly half the number off light at any one time and half again for time off day but even that is £900 a year with leds you can remove 85% off the costs down to £540/135 a year and would expect a further 25% saving if downlighters and spots are changed for pendants
All essentially true, but I suspect that the actual consumption is probably far less than you are assuming. About half of his total potential lighting loads relate to bedrooms/bathrooms/dressing rooms, where the lights are probably only on for an hour or two per day, at most, or places (like loft and spare bedroom) where the lights are probably hardly ever used at all.
Whilst there are clearly considerable savings to be had by changing to LEDs (or CFLs), and/or by using more efficient/sensible light fittings, I doubt that those savings would be anything like as great as your figures imply. As others have said, the one room where things currently seem rather ridiculous is the kitchen (I suspect a 'kitchen/diner'),which currently accounts for over a third of the total potential lighting loads - and, particularly given that
those lights are probably on quite a lot, there is a lot of saving to be had by addressing that room alone.
My kitchen/breakfast room has what, at least traditionally, appeared to be extravagant lighting, with 16 candle bulbs, but now that they are all 3W LEDs, that's less than 50W in total, and extremely light!
Kind Regards, John