Efficiency as in converting electricity to photons. 100 watts of 45% efficient light gives 45 watts of light, rest is heat. So, the higher the efficiency is, the less power you have to use for usable light
Thanks! that was understandable

But why would not everyone go for 100% efficiency? Only because of cost reduction? (buying fewer leds?) I mean, the way i see it... going for 100% efficiency is a higher investment, but pays out in the long run rite?
But if i want to build a equilavant of 1200 watt HPS, and i choose 12 LEDS that only run at 45% efficiancy... i only get 540 watts of 100% efficiancy?! how does this works?
Also, can anyone explain to me why Cree CXB3590 COB LED should be the best choice at this moment?
and what about this?
So a Cree CXB3590 COB LED chip can be run as high as 110W
What do you guys mean by can be run as high as?
I guess what you guys mean is that you can run the led on different watt's rite?
How do you do that? What else do i need beside LED's?