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Wattage is partly a remnant of the HID (MH and HPS) days. It used to tell you exactly how strong a light is, what area can be covered and so on, but growing with LED is completely different. For example: a 600W HPS light can have lower yields/results or PPFD measurements than a LED light with a 400W power draw. Therefore, wattage should not only be used to determine the required wattage or usefulness for a certain coverage area!
Wattage should only be used as a simple method for approximating the correct fixture size for your grow(environment).
If you only want to use wattage to determine a fixture's footprint or potential, keep in mind that the most efficient grow lights on the market require 32 watts per 30cm². So assume a minimum of 32 watts per 30cm² (for flowering). Less efficient fixtures (often manufactured in China with a lower light intensity and a higher power draw) require that you either use a smaller coverage area or add supplemental lighting to increase the intensity.
Also note, you should never use wattage to determine how much useable light an LED grow light emits. Wattage can vary dramatically between fixtures and brands: There are LED grow lights that will cover a 4.5'x4.5’ area that only draw 515 watts, and fixtures that cover a 4’x4’ area that draw 760 watts. Ideally, you want the fewest amount of watts with the largest footprint. The only problem is that a manufacturer can say the light covers a 4’x4’, but if the consumer does not know the Photosynthetic Photon Flux Density (PPFD), also known as light intensity as far as a plant is concerned, this statement is useless.
Another issue with using wattage to determine a fixtures footprint is that all of the wattage drawn does not get converted to light. Some power is lost to the driver, cooling fans (if present), and the rest of the energy is lost as heat.
In sum, it is best to not use wattage to determine a grow light’s intensity, ability to grow your plants, or coverage area. So what should you use? Use PPF and PPFD.