Monday, 21 November 2011

Should I buy LED bulbs?

A friend remarked to me the other day that LED lights are cheap enough now that they can pay for themselves (replacing a halogen lamp) within a year, at least for for use cases where the lights were on most of the time, as in many shops and offices. I checked this out and it is true. However, what about replacing a CFL bulb  (the usual energy saving light bulbs). How much less power does the LED take, is it really cheaper taking into account the high up front cost and is it really greener overall?

Even the power consumption question is not so easy to answer. LEDs are normally compared to halogens rather than CFLs or incandescent. For my calculations I have compared (both from Amazon):

  • 6W LED allegedly equivalent to  a 50-60W halogen (1) (£60 for a pack of 6 so £10 each)
  • 18W CFL allegedly equivalent to an 80W incandescent (2) (£12 for a pack of 2 so £2  each)

Based on electricity costs at 12p/kWh, the LEDs take 5000 hours of use to pay back the higher cost of the bulb compared to CFLs. At 3 hours a day that would be about 4.5 years. The LEDs should last a lot longer than that - a typical rated lifetime is 20,000 hours. The CFL should also last a lot longer - typically 10,000 hours or more. However, the CFL may not last that long if you only use it for short periods at a time. The performance on short cycle times is very variable and judging from this study, which, granted is using rather old data, a 15 minute cycle time could reduce the lifetime by three quarters or even more. Every time you switch a CFL bulb on and off the sputtering loses some of the emissive coating on the electrodes. However good design can minimise this effect and some bulbs did fine even with the short cycle time.

An LED bulb with aluminium (heat sink) casing
As for whether or not LEDs are really greener, this study for DEFRA  (from 2009) reckons not, though they are both much greener than incandescents. There wasn't much difference in the greenhouse gases emitted but the LED bulbs were rated worse on most other aspects, especially leaking poisons from landfill, largely due to the amount of aluminium used in the bulbs. (High concentrations of aluminium in water can kill plants and fish). However, I find these conclusions suspect. In this study the LEDs analysed were  12W worth weighing 280g (of which 100g was aluminium). However, the 6W bulb in my comparison only weighs 70g. LED technology is improving fast and I suspect the DEFRA study is already out of date and overestimates the environmental impacts of the bulb construction considerably simply because the bulbs are physically smaller and therefore contain less metal. In any case, if you dispose of the bulbs responsibly the aluminium can be recycled. Judging by this the LEDs are probably already greener than CFLs in overall environmental impact.

There you have the answers to the three questions I posed above. LEDs use something like a third the energy of CFLs, they are cheaper if you take a long term view and they are probably greener overall. However, there are other reasons to prefer LEDs to CFLs. We have some in our house and we love them mainly because the light looks bright and comes on instantly. The trouble with CFLs is that when you turn them on it takes a while to come on to full brightness. That one issue is enough to swing the balance for me and I don't think I will ever buy a CFL again if I can afford the LED.  Also, we don't use dimmer switches but if you do then you will be relieved to hear that you can get LEDs which are compatible with normal dimmer dials and use not too much extra energy (certainly a lot less than equivalent CFLs).

This is quite a complicated post so I have summarised the main points below.

Advantages of CFLsAdvantages of LEDs
  • Cheap up front cost (e.g. £2/bulb). It takes about 5000 hours of use for LEDs to pay back the difference in the cost of the bulb.
  • Although the lifetime is not as long as LEDs it is long enough for most purposes - 10,000 hours is more than a year and most lights will only be used for a few hours per day. At 3 hours per day 10,000 hours is 9 years.
  • Uses about a third the electricity for the same brightness
  • Achieves full brightness instantly
  • No problem with switching on and off lots of times - if you only switch the CFLs on for a few minutes at a time the overall lifetime (in terms of on-hours) can be significantly reduce.
  • Very long life e.g. 20,000 to 50,000 hours (so you don't need to get out the ladder so often to replace them)
  • Can be compatible with standard dimmer switches without much loss of efficiency  - CFLs that are dimmable usually have a different mechanism controlled by turning them on and off in a short sequence; they are not compatible with the wall dials we are used to.

(2) 6 x 18watt - now showing as equivalent to 82w conventional bulb
(3) Welcome to the Dark Side: The Effect of Switching on CFL Measure Life  (2010)
(4) Life Cycle Assessment of Ultra-Efficient Lamps (2009)
(5) Auraglow 7 Watt LED GU10 Light Bulb, Warm White, 50 Watt Equivalent (dimmable)


  1. Very informative and thorough, pity there are not more articles like these on the web - thanks

  2. Which is more efficient, running LED bulbs from 240v or running them from a 12v supply from say a switch mode power supply and how does the equation stack up when you include embodied energy?