in this context, watts are meaningless. 250 watts from what light? T5? Burple Chinese lights? COBs? High end commercial light like Fluence? a light like my Quantum Board at 250 watts at the wall will far, far out perform all the burple lights and many of the commercial ones also. And still, the power of the light is less than half of the equation. genetics, growing medium, organic soil or bottled nutes, brand of nutes, water quality, ph, temperature, RH, and airflow to name a few, can all affect final outcome. I've got a tent packed full of big plants right now. But my lack of experience with plants this size is costing me bud. I'm guessing I'll end up with 12 ounce from the three plants, when I figure 18 ounces is what I would have got with more plant training/defoliation.
on average: ( average not peak )
250 watts T5 : .4 gpw
250 ( true ) watts burple led: .6 gpw
250 watt HPS: .6 gpw
Top quality commercial LED: 1.5 gpw
DIY and commercial COB lighting: 1.4 gpw
But to hit even 1 gpw, everything must be right and stay that way the entire grow