LED Wall-Plug Luminous Efficacy and Bogosity, Part 1

The power factor may be the smoking gun that blows a hole in some claims about luminous efficacy.

We've all seen it, cited it, bragged about it, and scratched out heads over it. LED efficacy. This little ratio of lumen flux output to electrical power input, blessed by the all-knowing gurus at US DOE and EPA, forms the core of the LED community's being, the argument that favors LEDs over CFL lamps, and without argument, over the lowly incandescent light bulb.

Figure 1

(Source: Vimeo)

Simple enough. But we need to be very explicit -- the electrical power input for this computation is expressed in terms of real power. Not a big deal for DC efficacy claims, as we'd find in most LED devices. Where it gets complicated is for computing the wall-plug luminous efficacy of almost any bulb or luminaire other than the incandescent bulb when the input is from AC mains.

In fact, I submit that the wall-plug computation of SSL efficacy as luminous flux output (expressed in lumens) in its ratio of real power should be banned. It's quickly becoming obsolete.

Here's why. Using this community's current favorite whipping boy, Nanoleaf, let's take a look at their lab measurement in Figure 1.

The relevant measurements you see in this integrating sphere setup are shown in Table 1:

Table 1

Volts In	215.6 Volts
Current In	0.097 Amps
Real Power	12.4 Watts
Power Factor	0.593
Luminous Flux	1643 lumens

So, we can compute the amazingly good wall-plug efficacy, as they did, arriving at 1643/12.4 = 132.5 lumens/Watt.

The marketers trampled the engineers in running out the door to tell everyone in the press. Engineers reading the numbers are still scratching their heads as to how such miraculous lumen numbers could be achieved. Part of that could be the measurement method, where it looks like white PVC pipe was used to point the instrumentation's detectors right at the LEDs, versus at the wall of the integrating sphere (Figure 2), yielding a very high computation of luminous flux. But this is speculation, not fact, so let's give the Chinese the benefit of being genius.

Figure 2

(Source: Vimeo)

Nanoleaf ran with the claim of its "Frankenbulb" having the highest wall-plug efficacy as the core of their Kickstarter campaign, coupled with their 1,600-lumen and 1,800-lumen claims. Cutesy whistles, clarinets, and clean video production, good old just-graduated Canadian-schooled front men supposedly based in San Diego for what I speculate is a large, established, Chinese company, lent credibility to their claims to the tune of raising $273,278 from 5,756 supporters. Awesome.

Everybody seems to have wanted to get a 1,600-lumen bulb that used 12 W of power, pretty much double the lumen output of most LED bulbs out there. Humanity finally gets out of caveman candela levels. As part of their campaign, the boys weren't shy and got rather clever in their graphics. Here's one of them (Figure 3).

Figure 3

(Source: Kickstarter)

At one glance, excited sponsors could see they'd save seven-eighths of the energy costs of a 100 W bulb, which agrees with Table 1 to amazing accuracy. 100/12.4 = 8.065 bulbs. Awesome.

But for one small detail. A future-facing detail that must be addressed, which we'll cover in Part 2 tomorrow.

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