It's the bulb that would use 400 watts, the ballast also uses power (given off as heat) but digital ballasts use less power that magnetic ones, and therefore run cooler. Magnetic ballasts can also make a bit of noise due to the way they work, but digital ballasts are silent. If you are willing to pay the extra then the digital is the way to go anyday.

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Loafy's new grow log...

I could buy my weed off the streets, but the government say that would help fund terrorism....

So I will grow my own and save the world!