How do the old and new (post-2021) EU energy labels for LED light bulbs compare?

Question

One of my light bulbs recently failed, for the first time in a few years, and I had to go buy a replacement. Usually I try to look for the ones with the best EU energy label rating, since not only do more efficient bulbs use less power, but they also generate less waste heat and thus are likely to last longer (not to mention probably being of better construction in the first place).

I was used to typical energy labels for LED bulbs ranging from A to A++, but this time I got quite confused, since about half of the bulbs on the shelves had energy labels like E or F, which I'm more used to associating with the old incandescent "heat balls". Curious, I did some math and realized that some of those "E" rated bulbs actually had better lumens / watt ratios than the A+ rated ones!

What I started to suspect at that point, and eventually confirmed by Googling, is that the labeling system is being changed right now, with the new labels being quite a bit stricter than the old ones especially for things like light bulbs, which are nowadays a lot more efficient than 10 or 20 years ago. Confusingly, though, the new "rescaled" labels look a lot like the old ones and there's apparently an 18 month transition period, until the end of February 2023, during which products with either new or old labels can be sold in stores.

So far, so good. What I couldn't easily find an answer to, however, is how do the new labels compare with the old ones? That is, for example, if I see two LED light bulbs on the store shelf, one rated A+ under the old system and the other rated E under the new system, which one of those is actually more energy efficient? Or do I just have to do the math myself?

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^{(I did eventually find what I think is the answer, but it involved digging into the EU standards and doing some non-trivial math, so I figured it was worth sharing. I'm posting my self-answer below, and I welcome any reviews, corrections and/or improvements to it, as well as any additional answers.)}

Answer 1

OK, based on some digging into the old and the new standard, and assuming I did the math right, here's what I think is the answer.

TL;DR: The old A++ label roughly corresponds to the new A–E (and maybe F) labels, while old A+ corresponds to new E–G. Anything below A+ in the old system falls under G (least efficient) in the new system.

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The difficulty with comparing the old and the new label categories is that they're defined differently:

• The new labels are defined based on the "total mains efficacy", η_TM, which is measured in lumens / watt. In the simplest case, such as a non-directional light bulb plugged directly into mains, this is calculated simply by dividing the light's declared "useful luminous flux" in lumens by its rated power consumption in watts.*

• The old labels, however, are based on something called the "energy efficiency index" (EEI), which is a dimensionless ratio between the rated power (with a correction factor for PSU losses, if not directly plugged into mains) and the "reference power" P_ref in watts, which is calculated from the useful luminous flux Φ_use (in lumens) based on a non-linear formula that I believe is designed to approximate the luminous efficiency of old incandescent light bulbs:

  • For lights with Φ_use < 1300 lm, P_ref = 0.88 × √Φ_use + 0.049 × Φ_use.
  • For lights with Φ_use ≥ 1300 lm, P_ref = 0.07341 × Φ_use.

What this all boils down to is that, for light bulbs emitting 1300 lumens or more, EEI = (13.62 lm/W) / η_TM or, conversely, η_TM = (13.62 lm/W) / EEI. But for bulbs that are not quite that bright (which is probably most of them — the brightest bulb I have currently in my home is 1055 lumens), the relationship becomes a quadratic formula.

Anyway, for bulbs of 1300 lm or more, where EEI is inversely proportional to η_TM, here's a quick table showing how the old and new labels match up (still assuming an omnidirectional bulb plugged directly into mains):

Old energy label	EEI ≤	ηTM (lm/W) ≥	New energy label
	0.06	210	A (most efficient)
	0.07	185	B
	0.09	160	C
	0.10	135	D
A++	0.11	124
	0.12	110	E
	0.16	85	F
A+	0.17	80
A	0.24	57
B	0.6	23
C	0.8	17
D	0.95	14
E	∞	0	G (least efficient)

Threshold values in bold are normative, from the standards linked above, while the thresholds in italics are approximate values calculated by me using the formulas given above.

For dimmer light bulbs the total mains efficacy (η_TM) required to achieve a given EEI value goes down. At about 685 lumens, the thresholds of the old A++ and the new E labels coincide, and for bulbs dimmer than this, even some that would be labeled F (and all labeled E) under the new system may qualify for the A++ label under the old system.

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^{*) For lights with an external power supply you multiply η_TM by 92.6%, and for directional lights you multiply it by 117.6%. For directional lights with an external power supply you do both, for a total scaling factor of 108.9%. Don't ask me what these factors are based on.}