TL;DR – When buying an LED grow light compare the actual power consumption at the wall to the stated or theoretical power consumption of the chips. the closer the two levels are the better.
I’ve put together little hydro systems for a couple of friends and relatives, and for some reason they don’t want to spend the time to make their own LED lights preferring to buy pre-made despite the fact that homemade are typically about the same price and a big pain in the ass to make.
Buying LED grow lights can be a pain as well, there are a lot of different styles and types each has it’s own set of Pro’s and Con’s. Obviously you want the most “Bang For Your Buck”, but it is very hard to compare the output of two different lights. Ideally the light manufacturers would agree to use a standard measurement and methodology but sadly they don’t. In the absence of a real standard the most important thing to be aware of is that when a manufacturer refers to a light as being a certain Wattage that statement probably has nothing to do with the actual power consumption and little to do with the brightness of the light.
The reason for it goes right back to the chip manufacturers themselves, leaving aside for the moment COB or Chip On Board lights, the 1, 3 and 5 Watt designations used on LED chips are, or have become, class descriptions not actual measurements of power consumption. The criteria that determines the class of the chip is the forward current at which it is driven, 1Watt chips are typically driven at 350mA, 3Watt chips at 700mA and 5 Watt chips are driven at 1200mA (1.2 Amp)* Manufacturers (or those doing the marketing) usually just multiply the number of chips in the light by the “Wattage” of the chips and refer to that as the Wattage of the light. (Though some have started to refer to these ratings as Model Numbers or Series Numbers in their fine print)
While some 1 Watt chips actually consume 1 Watt it is often impossible to reliably drive 3 or 5 Watt chips at this stated wattage. (See Table 1.) Neither Table 1 nor Table 2 attempt to represent every chip from every manufacturer, these are typical or average values only.
[EDIT Feb. 12 for clarity: I don’t mean to say that no chips actually consume the power they are rated for, some can indeed be driven at a high enough current to make their rating but they are regrettably not typical.]
Unfortunately while these numbers are already misleading the manufacturers don’t even power the chips to the full forward current that determines that chips class. By purposely under-powering the chips the manufacturers reduce the heat generated, increasing the chips reliability and extending their life. Under-powering the chips would also allow a manufacturer to use inferior quality chips should they so choose. [To be fair the often most efficient way to drive a chip in terms of light produced / energy consumed is to drive it at a point bellow the rated current, but since efficiency is not reflected in any of the marketing I wont be addressing it.]
Ideally grow light manufacturers would list the forward current at which they drive their chips, and while I’ve seen it listed it is very rare. In most cases in order to accurately evaluate a light you may be thinking of purchasing you should dig a bit deeper in to the spec. sheet. Most manufactures will list the actual power consumption of their lights, as general rule of thumb (and this applies to all types of LED light) the higher that number is, as a percentage of the claimed wattage, the better. Even so, don’t expect to see values near those found in the descriptions. As shown in Table 2 (Bellow), a hypothetical grow light using 100 3Watt chips or 60 5Watt Chips, and having a 5.5:1 Red:Blue ratio might have a typical consumption of around 160Watts, assuming chips were driven at the Typical forward current for their class.
Currently on ebay you can find two virtually identical “300 Watt” grow lights, one with an Actual Consumption listed as 140W +/- 3% and one listing 100W +/- 3%, that represents either 87.5% or 62.5% of the theoretically available power.
Of additional interest…
Different Colour LED chips are made from different materials and have different potential Forward Currents, a good wikipedia article and a table listing each can be found here:
* 5Watt chips are even more confusing, some as above, rely on a higher froward current but some are built like the COB (Chip On Board) lights with two or four emitters on the chip. The result is a higher forward voltage or forward current created by tying multiple emitters together in parallel or in series.