LED grow lights utilize LED technology to grow indoor plants and other greenhouse crops. LEDs offer many benefits that traditional grow lighting, such as high-pressure sodium, metal halide, and fluorescents, do not provide.
Generally, 30-50% less wattage is needed depending on the efficiency of the LED grow light. More efficient grow lights exude more light while using less watts. Remember, that this is actual LED wattage, also known as wall wattage, not the advertised LED wattage that might be found in the title of the LED grow light. For example, an advertised '1200W' LED grow light might draw only 500W at the wall.
You will want around 32 watts of actual wattage per square foot of grow space. Some LED grow lights require less, while others require more depending on the efficiency. It is best to follow the accompanying recommendation because all LED units are designed differently.
You shouldn't actually use wattage to determine how powerful your LED grow light needs to be. Instead there are other factors that are more important, such as PPFD and footprint uniformity.
PPFD (photosynthetic photon flux density) is a measure of the number of photons in the 400-700 nm range of the visible light spectrum that fall on a square meter of target area per second. High PPFD numbers are good, because this means your plant is 'absorbing' enough light to carry out important physiological processes. Keep in mind that there are LED lights that have high wattages, but could have low PPFD numbers due to many factors such as: wide beam/lens angles, large distance between LEDs, an increased height above the canopy, low efficiency, and so on.
The fixture's average PPFD will tell you how much light the unit is putting out over the specified area.
Recommended distances may vary based on the brand, light shape, coverage area, beam angle, etc. These factors all affect the Photosynthetic Active Radiation (PAR) that falls on the plants leaves. The best advice is to follow the product recommendations as they will all be slightly different. But generally, 18"-36" is a typical distance: 18" for flower and 24" or more for vegetables.
This is not recommended. Since the plants are used to a particular light intensity and spectrum, switching suddenly between lighting sources could shock the plant and delay flowering. It is not unlike growing a plant indoors and placing it outside for the first time without gradually exposing it. It is more assuring for the plant to use the same lighting system.