To keep it short 400 watts of light is 400 watts of heat + or -. The differance is the amount of space given to dissipate that heat. One 400 watt MH fixture/reflector is around 72 sq. in. Where as one 150 and one 250 would be two fixtures/reflectors around 144 sq. in. Same thing would apply to T5HO 8 x 54 watts = 432 watts but this is normally spread out in 558 sq. in.