it depends on the strain,size of the plant,how much room you have,etc. a 400 watt will supply enough lumens for a 4 or 5 ft. x 4 or 5 ft section.i'd recommend a 400 watt over a a 250 watt anyday.theyre well worth the money.insidesun.com,hydrowholesale.com and htgsupply.com are all great websites if ordering online.i just got my two 600 watt hps growlight after only waiting 2 days from hydrowholesale.if you have the money,run a metal halide for veg,and a hps when its time to flip the lights to trigger flowering.if not..a hps can be used throughout the grow,they just might stretch while in veg so it important to know about supercropping. hope this helps ya out-peace