LED Wall-Plug Luminous Efficacy and Bogosity, Part 2

Would you believe that an LED lamp with a claimed luminous efficacy higher than 100 lm/W could actually be as inefficient as an incandescent bulb when dimmed? Believe it.

In part 1, we validated Naonoleaf's numbers, which are not really any different in computation than what anyone else uses, though we still questioned its lumen flux measurement methods (and the identity of its backers).

Real and apparent power

Your old electric meter measures real power, but it won't do so for long. The newer smart power meters do not bill real power used. Instead, they do a computation of apparent power, which is simply a product of line voltage times current. There are significant costs and infrastructure overdesign required to deliver reactive power -- anything with a power factor of less than one. Once again, MBAs have figured out how to get revenue from something imaginary, and should we be surprised?

For incandescent lighting, the smart meter switchover didn't matter, because that is a purely resistive load with unity power factor. But things like motor loads from refrigerators, dryers, and furnace blowers benefitted from real-power measuring meters. Consumers were outraged when their electric bills more than doubled, most likely because newly installed meters were computing V*I, not real power.

If smart meters are in everyone's future, the classical wall-plug efficacy computation using real power will be bogus. Consumers are no longer paying for real power. They are paying for Volts*Amps, or more precisely for kVA (interestingly, at the same rate per unit as they were for real power).

The Bogosity Coefficient

When we look at classically computed wall-plug efficacy in the presence of smart meters, we have to apply what I'm calling the Bogosity Coefficient to the SSL's claimed power use. Alternatively, we can divide by this coefficient to figure out the amount of money saved. The Bogosity Coefficient is easily calculated -- it's simply 1 divided by the power factor.

The sponsors of Nanoleaf are not going to be paying $50 for their power bill versus $400 for the incandescent and $100 for the CFL. Rather, on a smart meter, they will be paying $50/0.593 = $84. For a >$30 light bulb.

Not only that, but we can apply the Bogosity Coefficient to the company's wall-plug efficacy claims. Again, it's irrelevant how much real power is being used when the consumer pays for kVA. So, Nanoleaf's wall-plug efficacy is really 1643/12.4/0.593 = 79 lumens/Watt. On a smart meter, this would result in the same electric bill as an LED with a power factor of 0.9 (1.11 Bogosity Coefficient) and a claimed (real-power) wall-plug efficacy of 88 lumens/Watt. In other words, you won't see any difference in your utility bill with its bulb versus pretty much any other LED bulb.

Here's one more thing that will blow your mind. User ratsbew over at the CandlePowerForums did some experiments with LED bulbs and wall dimmers. Using his numbers, your 88 lumens/Watt LED bulb will actually have a smart meter efficacy, when fully dimmed, of about 17 lumens/Watt -- the same as an incandescent bulb.

Related post:

• LED Wall-Plug Luminous Efficacy and Bogosity, Part 1