John Bullock Lighting Design
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LEDs - now this is getting complicated!


Unless you've had some sort of training in electrical principles, this may get a bit awkward, so I'll try to keep it simple. But there's trouble brewing and I think we should get this out in the open - and it's about LEDs.

In the days of my youth, when we had Industry in this country, I was introduced to the philosophical concept of Power Factor - and that's what this piece is all about. An understanding of Power Factor only requires you to believe that energy can exist where no one can see it - on top of the simpler task of believing seven impossible things before breakfast.

There are plenty of webpages dedicated to explaining Power Factor, so I'm not going into the gory technical details; suffice to say that some electrical components (induction and capacitance - that kind of thing) can consume energy in a completely different way to the way that practical stuff like kettle elements and electric hobs do. Its all comes down to 'useful' electrical consumption and 'wasted' consumption. (my terms - you won't find them on any of the technical sites, but they work for me)

So how good's your relationship with the cosine law?  - because that's what's at play here. Think of the 'useful' current as being the vertical axis of a graph, and the 'wasted' current being the horizontal axis. You measure the two currents, and join the lines together to make a triangle - the 'total' current is the hypotenuse of the triangle. Got it?

If you've got no 'wasted' energy - like you get with a filament lamp, because its all useful resistance, then the 'total' current is the same figure as the 'useful' current - it has a cosine value of 1 (and has a Power factor of 1). A 20W lamp uses 20W of electricity. But if its a fluorescent circuit, say, and the 'useful' current measures the same as the 'wasted' energy, then it will create a 45 degree triangle and the cosine figure (Power Factor) will be 0.707. The energy consumption of a 20W lamp would actually be 20 / 0.707 = 28W, an energy lift of 40%, which is quite a lot, don't you think.

The LED industry is getting ready to be beaten with the Power Factor stick because a lot of the lamps in production - and in particular, the retro-fit lamps intended to replace tungsten halogen reflector lamps - have a Power Factor of only 0.5 (and some poor lamps have even lower PF figures). So the 5W lamp that you've presented to your client and worked out the cost comparison  to show how quickly (or not) the high capital cost of the lamps will be recouped will be out by 100%, because the thing you need to do is divide the published 'useful' energy rating of 5W by the Power Facor (0.5 in this case) to produce the 'total' energy figure - and that makes it a 10W lamp.

So next time you're specifying LED replacements make sure that the manufacturers of the lamps can give you the full data. You'll find that you'll still reduce your client's energy bills, just not so quickly.

Just thought I'd mention it.

 One final thing - is there anything the LED manufacturers can do about it?

Oh yes. They can produce high power factor versions of their LED lamps - if they want to. They may cost a bit more, and they may involve a bit more technical development, but yes, it can be done.

RIBA CPD in 2015

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