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I have ordered a growth light for my plants. Only later did I realize that it said its a 2kW light, which seemed quite excessive as it is only the size of a small keyboard and not that densely populated with LEDs. So I did some digging and figured out that it is using 2835 model LEDs. I found a website where I can buy those "by the meter". The website states that with 120 LEDs/meter it will use 24W/meter. My growth light has 216 LEDs. Lets round that up to 240. This would mean it uses 48W and not 2kW. What could be the cause for that difference? Is this related to the equivalent for a non-LED based growth light?

I looked up some generic LED light I can buy in a store near me and there you can also find some weird wattage conversion:

Image of a package of an LED bulb where it says 4.6 watt equals 35W watt

The 4.6W = 35W is what I am talking about.

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    \$\begingroup\$ The cause is probably marketing people. \$\endgroup\$
    – John D
    Oct 6, 2020 at 20:04
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    \$\begingroup\$ I'd go a step further and say it's unchecked greed. \$\endgroup\$
    – Marcus Müller
    Oct 6, 2020 at 20:07
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    \$\begingroup\$ @YanickSalzmann if it was doing 2 kW, you will need to turn over your chicken after 15 minutes to achieve perfect crispiness. This is marketing nonsense, no matter how many vendors jump on the bandwagon. \$\endgroup\$
    – Marcus Müller
    Oct 6, 2020 at 20:08
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    \$\begingroup\$ Even if they're comparing light output of LED's with incandescent lamps, the ratio is about 8:1 or 10:1, so 48 W of LED's might be the equivalent of 500 W of incandescent lighting, at the most. As mentioned, 2kW would cook the cabbage and pop the corn. \$\endgroup\$
    – DrMoishe Pippik
    Oct 6, 2020 at 20:26
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    \$\begingroup\$ That 4.6W = 35W is based on supposed light output vs Incandescent bulb power usage vs LED power usage. But its also important that you get quality leds and not some fake garbage that says it's a 35 watt equivalent but really isn't. \$\endgroup\$
    – Passerby
    Oct 6, 2020 at 20:28

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Only later did I realize that it said its a 2kW light, which seemed quite excessive as it is only the size of a small keyboard and not that densely populated with LEDs.

Low quality grow lights are advertised by their supposed incandescent equivalent wattage. Usually if you look through the listing, they'll also give you the wall power. However, lights like these usually use cheap, low efficiency diodes, so even with the wall power draw it is hard to know how much light you're getting.

Unless you only need to use the light briefly, it is usually a better idea to pay for higher efficiency LEDs. If you do that, you'll get a specification for mols/s of photons per watt from which you can calculate the actual light output and thus size your light more accurately. In the long run, high mol/j lighting is usually much cheaper unless you have very inexpensive electrical power.

So I did some digging and figured out that it is using 2835 model LEDs. I found a website where I can buy those "by the meter". The website states that with 120 LEDs/meter it will use 24W/meter.

Watts per meter refers to strip lighting on a flexible substrate. More likely you have a metal core PCB grow light, in which case you can have higher power per LED, so this isn't necessarily correct.

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