Posted on 16th October 2015
LED lights seem a no-brainer: instant on, good colour reproduction, extremely energy efficient (cool to the touch after minutes of use). But they are expensive. I wonder what the payback time is?
My electric supplier charges 10.98 pence per KWh. In winter, we heat the house, so lets make the assumption that the extra electricity an incandescent or compact fluorescent bulb uses needs to be made up by the gas fired central heating. Let's further assume the boiler and system are 75% efficient. My gas charge is 3.69 pence a kWh, so 4.92 pence once we consider efficiency loses. I'll assume I run the heating about half the year; so half the year the lights save gas costs, and hence a KWh less of electric use will only save 6.06 pence.
As an aside, the cost per KWh means that using one watt continuously for a year costs one pound.
I recently replaced an 11W CFL in the bathroom with a 5W LED (which continue to fall in price, if you shop around). Assuming it's on for an hour a day (how long do you spend in a bathroom) that's 18.6 pence a year saved. Payback time: 27 years. And 5W isn't quite enough...
Let's consider the kitchen: 4 CFL bulbs of 12W each. Let's suppose we're cheap and replace them with 6W bulbs. In summer, maybe the lights are on 2 hours day, and in winter, 5 hours a day. So in summer we save 8.76 kWhs and in winter, 21.9 kWhs. Or £2.29 a year. Payback time: 7 years.
My parents still use incandescent bulbs they have procured from somewhere. However, they have electric heat, so for a good six months of the year, the "waste" from the bulbs isn't really wasted at all: it's simply the electric radiator being used less. So a 60W bulb replaced by a 10W bulb, and used 4 hours a day, would say 36.5 kWhs a year, or £4. Pays for itself in a bit over a year.
Not worth it for energy saving alone; but worth it for the instance on and better light quality.