That is a question a lot of people ask. Actualy when it comes to light people who use hps/ cfl people use lumens when it comes to LED people use par value/umoles. Parvalue and umoles is the same thing just different term. Parvalue is the most accerent way of determining the power of the light that the plant actualy uses. But for HID and cfls most people can't offord par meters but the manufacture suplys lumens so that's why people use that. LEDS are such new tecnology for light that manufactures are up 2date and use par value for people. Now par value is a whole other thing to figure out but I can help ya. U need to find the par value at what distance ur gunna use if ur gunna use that bestva 800 the par value and distance in my opion should be. Kept at 6inchs above the plant as this is the distance I use my bestva at and is just fine and works great all from seedling to flower now the par would be. 1726 now to figure out ur true par is to take seconds per min times by mins per hour 60x60 equals 3,600 second per hour now times that by how many hours u will run it so I run mine 24/0 then if I grow photos when flower comes I switch to 12/12 but if I grow autos I leave it 24/0 for the full grow. So take 3,600 seconds per hour times by how many hours 3,600x 24 equals 86,400 seconds per day now times that by ur par value 86,400x1,726 equals 149,126,400 now this equals umoles now u need moles of light when dealing with LED so there's 1million umoles to 1mole so u take 149,126,400 and devide into 100,000,0 equals 149.1264 round down 149 moles of light per 24hours now if ur doing photos cut that number In half from 24/0 to 12/12 so ud have 75moles. Now u want around 150-200moles of light per day 200moles would equal a 1000watt hps roughly. But I do not know how to convert lumens to par so that I can't help and if anyone nows how to do it I'd love to learn lol