god this is posted so much, I've answered it 4 times in the last week lol.
I've allso allways said this. Electronics say the max watts, yeah a pc may have a 600w PS but only uses a few hundred most of the time. The difference with grow lights is their usage is constant.
15 cents a KWH? holy fuck, I thought I payed alot for power at 7 cents lol.
Anyway look at your power bill and find out how much per KWH
then do this,
WATTS x 18 x 30 DIVIDED BY 1000
450 18hrs days = Your Monthly KW used. 243 in this case.
Multiply that by your KWH price and thats how much it will cost to run your light.
Example: 243 KW x .07 Cents = $17.01
I wouldn't worry about it, yeah try to shut other stuff off when ur light are on.
If your going to add another light, I suggest a 600 watt hps for flowering, the 600's are much more efficient than the 400's and don't cost much more.