Because LED technology is so new, it is often hard to understand the difference between the multitude of lights available.
Many LED grow lights contain 1-watt (1W) or 3-watt (3W) LEDs and claim to be the "most efficient". This is true in the sense that a 1W LED is more efficient than a 3W LED at producing light, and a 3W LED is more efficient than a 5W LED, in terms of photons produced per watt of power used. However, a 0.5W LED is even more efficient than a 1W LED, and the most efficient demonstrated to date is a 30 picowatt (0.00000000003W) LED, so in this sense the 1W or 3W chips are certainly not the "most efficient".
So why aren't LED grow lights using the most efficient 0.00000000003W LEDs? As the power of LEDs decreases, so does their intensity. A 30 picowatt LED puts out so little light that you probably wouldn't be able to see if it is on or off, and you certainly can't grow a plant with it. 1W LEDs are much brighter, but still relatively dim- they're about as bright as an old-style T12 fluorescent (1.5-inch-thick) tube, but only about 1mm in size. They are intense enough to grow plants with, but only within a few inches of the LED.
3W LEDs are brighter still-- a little brighter than a new-style T5 (⅝" thick) fluorescent tube, and bright enough to grow plants within 12-24 inches of the LEDs. Black Dog LED was the first company to offer an all-single-die-3W LED grow light in 2010. The 3W LEDs were very capable of growing and flowering plants and marked the true beginning of practical LED grow lights. Our experience with these lights left us wanting a little more canopy-penetration power; they just weren't intense enough to keep lower leaves healthy on large plants. When the technology became available, we experimented with 5W LEDs and found them to be even better for canopy penetration and growing plants, which is why our Universal Series lights use exclusively 5W LEDs, and we no longer offer 3W LED grow lights.
5W LEDs are intense enough that it is uncomfortable to look at them (and they may damage your eyes!), more like the sun, MH or HPS lights. They have the power to penetrate the plant canopy and keep lower leaves healthy and contributing to the plants' growth. (In our office growing chamber, we have plants robustly growing under another upper layer of plants, more than 4 feet from the Black Dog LED lights!) The more leaves a plant can have contributing to its' growth, the better the plant will grow. Many growers are familiar with the concept of "pruning up" lower leaves under artificial lights; with 5W LEDs the need to do this is dramatically reduced or entirely eliminated.
Other LED grow lights are still using 1W and 3W LEDs because they are much less expensive and much easier to cool. 5W LEDs require specialized thermal management to keep the LEDs from burning out quickly. It is cheaper and easier to design and manufacture a grow light with 1W or 3W LEDs.
If 5W LEDs are great for growing plants, it seems as though 10W LEDs would be even better, but this is not (yet) the case. 10W LEDs are not available yet in all the colors we need to create our Phyto-Genesis Spectrum™. The 10W LEDs that are available are primarily "white" LEDs, which are great for creating lights for humans, but aren't very efficient for growing plants, as we explain here. They actually put out less light that is usable to the plants than our existing 5W LEDs, while taking twice the power!
Chip-On-Board (COB) or "Integrated" LEDs are created by placing many low-wattage (typically 0.5W or 1W) LED dies side-by-side on a circuit board. This allows companies to claim very high wattage LEDs, but for example the "50W LEDs" are just 50 1W LEDs side-by-side (or 100 0.5W LEDs). It isn't possible (yet) to make long-life COB LEDs with higher-wattage dies, as it isn't possible to remove the heat from the COB assembly quickly enough to prevent damage to the LEDs. For example, there are now some COB LEDs made with 3W dies on the market, but these are still only run at 1 watt per die and still have a tendency to burn themselves out within a couple months. COB LEDs also often lack primary lenses on the LEDs, which actually diminishes the light output from the LED.
Efficiency is incredibly important, but it doesn't matter if your LED grow light is the most efficient if it won't grow plants well. Black Dog LED's Universal Series lights use only 5W high-power LED chips in our proprietary Phyto-Genesis Spectrum™ to provide the best combination of efficiency, spectrum and plant-growing power.
Source: Black Dog LED