Explainer: why Power Factor is important in an LED luminaire

WARNING: electrical talk here!

Power factor is all about wasting energy.

The LED chip is only one part of a complex electrical circuit. Some of the components within the drivers do strange things with electricity, like consuming power that only gets noticed at the point of generation. It doesn’t register on domestic or commercial meters, but it’s burning up energy nonetheless.

Here’s an old-fashioned image:

Imagine a horse pulling a barge along a canal. The ‘useful’ power from the horse is what moves the barge forward along the cut. But there’s something else going on. Unfortunately, the horse can’t walk on water. Instead, it uses the designated towpath which runs alongside the canal. But that puts an angle into the direction of the pull, so the bargee is obliged to operate the tiller in order to keep the barge parallel to the canal. The tiller applies a frictional drag, so the horse has to work harder. Anyone watching our heroes progress along the canal will only see the forward motion of the barge, the energy required to counter the tiller is ‘invisible’ – but the horse certainly knows all about it.

The last horse-drawn barge on the River Lea – in 1955 near Waltham Abbey
Thanks to Chelmer Canal Trust

The drag caused by the tiller is analogous to the ‘invisible power’ being consumed in an LED circuit.

It doesn’t have to be this way. If the bargee has a longer tow rope, s/he can reduce the drag on the tiller by having the horse much further forward along the towpath – so less horse energy is wasted. ‘Thank you’, says the old ‘oss. For the LED driver, all it requires is a few more components to counter the effects of the components causing the problem.

The Power Factor Triangle

In electrical terms, the ‘useful power’ is the ‘Real Power’. The combined effect of the ‘useful power’ and ‘invisible power’ is the ‘Apparent Power’. It’s a triangular relationship, as per this graphic.

LED circuits should have a Power Factor of .0.9 – ideally it would be 1.0, but that’s very ambitious.

An LED fixture can have a Power Factor as low as 0.5, sometimes even lower. In such a case, the LED circuit will consume TWICE as much energy as the rated wattage of the luminaire.; so a 9W fixture actually consumes 18W.

There are three reasons why this is important:

Firstly; there’s an ethical argument. The lost energy of an inefficient LED circuit is paid for at the generating plant and the cost of that energy is then paid for by everyone buying power from that source.

There’s the off-grid argument: for the increasing number of off-grid installations, or installations using battery storage, that invisible energy can make the difference between a system that is balanced and one that is overloaded.

And a very practical point: although an ordinary meter won’t register the invisible power, it will affect the amount of current flowing through cables. And if we consider the growth in the use of 48v systems and Power over ethernet installations, that extra current can have a devastating effect on the cost of wiring. Current flow also impacts voltage drop, of course.

ACTION: always check the Power Factor of many LED fixture that you’re planning to specify or install. There’s no excuse for a Power Factor of less than 0.9.

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