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How to choose the best low energy lighting

Posted by Cathy Debenham on 9 May 2012 at 9:14 am

Traditionally we've measured the strength of our lightbulbs in watts. A 40 watt incandescent light bulb was dim, a 100 watt one was bright. However, that measure was misleading. Watts are a measure of power consumption, not of light.

A 100 watt incandescent bulb needed 100 watts of electricity to light it. As a result it was brighter than a 40 watt bulb. To get the best low energy lighting, we need to know more. The goal is to get the right brightness, for the lowest energy use. So this is a guide to the measures you need to be aware of when you buy low energy lights.


Lumens are a measure of the light given out by a lamp. The higher the lumens, the brighter the lamp. A traditional 40 watt bulb gave out around 450 lumens. As a general guide:
Incandescent lamps give out 10-12 lumens per watt
Compact fluorescent lamps (CFL) give out 50-60 lum/w
Light emitting diodes (LED) give out fomr 40-90 lum/w

Colour temperature:

The colour of light is measured in degrees Kelvin. The colour is what results in us perceiving light as warm or cold; harsh or soft. Cold light will have more blue in it and warm white has more yellow.
Daylight is 5,000 Kelvin
Cool white is 4,200k
Warm white is 2,700k
In the UK, most lamps are between 2,700k and 3,500k.

There is another colour measurement, the colour rendering index or CRI. This measures the quality of a light source compared with sunlight. Sunlight is given the maximum CRI value of 100. The closer a lamp is to that, the better its ability to show true colours. This is important in art galleries and shops, but is not critical for optimal light levels in homes. Lamps for domestic use can have a CRI level as low as 60 (which means they are less expensive to produce and buy).

Energy use:

On the packaging this is measured in two ways. There will be a number of watts. This tells you how many watts are needed to power the lamp. The lower the number, the better for your purse. There will also be an energy rating, where A is the best, and G is the worst.

See also this blog about comparative costs of lighting



If you have a question about anything in the above blog, please ask it in the comments section below.

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4 comments - read them below or add one


malcolmchisholmComment left on: 28 April 2013 at 2:17 am

The transformer that powers your 12v halogen lamp will usually be designed to switch off if insufficient power is being taken (as when the halogen lamp fails).  When you replace a halogen lamp with a LED, the transformer will have much less power taken from it.  This may cause the transformer to shut down.


You can try powering all your LEDs from one transformer, up to the ratied current for that transformer, or you may need to buy one new transformer to power a number of LED lamps.

Alternatively, buy GU10 fittings, which use 240v LED lamps and don't need tramsformers.


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Cathy Debenham

Cathy DebenhamComment left on: 25 May 2012 at 8:25 am

@Diomedea Exulans: If you read through the comments under this blog you'll find more information about this subject.

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Diomedea Exulans

Diomedea ExulansComment left on: 24 May 2012 at 6:18 pm

Some years ago we installed a lot of 12v halogen downlighters, which were recommended as efficient at that time. But times have changed, and we should now like to get LED downlighters instead, at about a tenth of the energy consumption. We bought a test bulb and tried it in one of the sockets; every time you turn it on it flashes and then goes off. A friend says that he changed his previous bulbs for LEDs and found that they worked really well, though there was a high percentage of dud bulbs. However, we've found a website which advises "for low voltage systems you will undoubtedly need to also swap out existing 12v transformers for constant voltage LED drivers. These can be ridiculously priced, but you should be able to get a perfectly reasonable product delivered to your door for about a fiver; this basic 10 – 50W LED driver sits in that bracket (the postage costs more than the item itself, which is not uncommon on low cost commodity products) and will power between 2 and 10 5W LEDs."; and we haven't done this, which sounds complicated and expensive (presumably you have to employ an electrician to fit these things). Maybe ours is a dud bulb. But the second explanation might account for why our bulb isn't working. Is it fact, or just a myth that firms use to get more business?

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utility-exchangeComment left on: 9 May 2012 at 10:15 am

Really interesting! Thank you!

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