30-40% is the number you are looking for.
Things you need to consider:
1. Growing under LED takes a bit of adjustment. Environmental, light positioning, lenses and spectrum all have to be considered. There will be a bit of a learning curve. Took me 3 grows to dial everything in once I had the light I needed.
2. Efficiency is offset by capital cost. While savings are pretty decent the initial cost of the lights is high and ROI is difficult to achieve in some cases. A theft or seizure of grow equipment would be devastating for a small operator. Whereas with HPS, you could be operational again within a day or two due to the low cost and ready availability.
So if you are running 600 watts HPS, if you go watt per watt and do 600 watts of LED, your yield should increase and your cooling costs decrease. If you want the same yield you get presently under HPS, shave 30-40% off the LED wattage; therefor a LED light with approx 450 watts (actual, not equivalent, ignore equivalents) should do the job for you. In this example your savings are on cooling (which you basically will no longer need) and electricity, again (obviously) to the tune of 30-40%. Growth characteristics of your plant may also change under LED, if your crop under LED is lighter/fluffy, the plant is stretching etc, it could be you dont have enough watts or the lights are too far from the plants. You will definitely see your plants do different things under LED. Buying multiple smaller LEDs is sometimes a better option than one big one that has to be raised up higher to cover the entire canopy. Smaller, overlapping lights give you better PAR values without hot spots and will provode some redundancy in the event a panel dies. (rare). Active or passive cooling is another choice you need to make and is primarily dependent on your LED light manufacturer and your grow space.
As for how many people are doing it, I have no idea, but it is catching on and most of the early adopter pains are over. Cost of the lights is still the primary issue. The tech keeps getting better every year. The low heat footprint makes indoor grows way stealthier, choppers with FLIR will not be able to see your grow glow from the air
I also learned it's a waste to use LED for cloning/rooting/vegetative growth (florescent works fine at a fraction of the cost), so go with a flowering spectrum, heavy in the reds or warm whites or both. The best part of LED is that you get to design your own spectrum.
Don't bother with UV LEDs, If you really want UV you will have to supplement with tanning bed floros or some other source. Always wear eye protection.
One last piece of advice, if you are going LED, take that opportunity to entirely re-engineer your grow. Reconsider how you do everything, cooling, odour control, humidity, circulation etc.
