600 Watt X 2 would give you 1200 watts or 1.2 Kw per hour. 1200 watts running on one circuity might be ok with the right gauge wire 12-2 grounded but would be taxing on smaller wiring, like most extension cords or such at 14 ga. or 16 ga. So I don't know your socket or wiring set up, and that makes a lot of difference.
Call me a cheap....sake. 2 X 600w = 1200 w o= 1.2 Kw. We in Michigan, on average pay $0.13 / kw per hour. that figures to be 1.2 Kw X 0.13 = $0.156/ hour x 16 hours of light (figure on timer) would cost $2.496 per day X 30 days would cost $74.88 per month, and this is on top of what it costs you to run your other household, ie lights, refrigerator, freezer, TV, computer, etc etc. I personally would not want to add an additional $75.00 to my electric bill. I have 2 each 40 watt (but actually figure out to about 36 watts) but let's say 40 watts for sake of simplicity. So my 80 watts is like leaving on a 75 watt light bulb all day and into the evening hours. I could not imagine the burden on my budget to run several thousands of watts (grow light) for long periods of the day. National average is 12 cents per Kilowatt consumed.
LED are so efficient in light received to electricity consumed that the winery where I work has converted all the cellar, lab, office and tasting room lights to LED. They look a lot like regular florescent lights, but smaller in diameter and about the same length. Full spectrum 5500K
It's a balancing act of light spectrum/ output/ costs / containment (note - containment is highly reflective sides and top and base to keep the available light more useful all the way down as far as you can go) Light without some sort of containment looses usefulness quite quickly in distance from the light source.... except for the sun.
