Well, 6x42W would be essentially the same price: the number of Watts determines the power draw. The difference is that with 250W HPS you get 26000 mean lumens, while for the 252W fluoros you'll have about 16000.

Running 24/0 at 10cents/kWh is about $18 / month, then flowering at 12/12 would be $9/month.

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Lumens, lux, and adding it all up :: 16K HPS vs. CFL :: DIY Drip Irrigation :: Pack water transport