Can of worms....it's not watts, its lumens. LED guys will tell you its PAR....maybe with LEDS' it is. But normally its lumens and you need about 5,000 lumens/sq ft. A 600w HPS is rated at 90,000 lumens. It's not actually going to stay at that value. It degrades pretty quickly to 80% of that within a week or so and by the end of 2 months it is at 60%. So the part of the equation that is missing is the area. If you are in a 4x4 tent then you are at 16 sq ft. So worst case scenario is 90,000 @ 60% = 54,000 / 16 = 3375 lumens per sq ft.