VHO=very high output, requires a special ballast,runs hotter than PC or NO but not as hot as MH,(especially with the electronic ballasts that are replacing the old coil and tar ballasts), usually installed in a hood or canopy with a small fan, provides twice the out put of NO's, provides the best color and florescence of animals,.......
I have 380 watts on my 40 gal reef. I'm very happy with my decision to go with VHO.
HTH
Hermit