this guy is bang on.. the formula is Power(watts) = Volts x Amps actually P(w)=E(v) x I(a)... watts, amps, volts.. are the units of measurement.. so we know our volts 120... we know our amps 20 or 15 on a lower case scenerio but we're gonna go with 20.. P=120x20 P=2400 watts on that circuit.. BUT you wanna use 80% of it(some say 85%).. that's how IAm5toned arrives at the 1920 watts constant over a 20 AMP circuit... If you wanna stay safe and have piece of mind.. BUT also that's why we invented breakers.. they are designed to trip if overused... Just hope your breaker works right.. Just thought i'd give a crash course on how to figure it out... and how he ended up with those numbers... it's the same thing for any volts or amps.. just punch in the numbers.. and you will get the max wattage.. then you multiply by .80 to get the 80% safe use number... It's a cake walk once you know the formula.. ANYWAY... those lights are 1000w X 2... if they are efeicent electronic ballasts then they probly use no more then 1100 watts each.. so 2200 watts total on a high.. (2100 in a best case scenerio) so regardless you're over your safety net BUT under your max.. you don't have much room for spikes.. which will trip the breaker and fuck your light cycle.. not worth finding out.. Like mentioned above the breaker will heat up over time.. it may not trip with those numbers BUT it will wear out and It may not work right over time.. WHICH is a safety concern... You could get away with it ...BUT it is not recommended and YOU'RE responsible for your actions.. The best thing to do here is run a second 20 amp 120 volt line.. and if your gonna do that, your better off putting in a 30amp 240 volt like mentioned by someone in this thread.. that's if you have room in the breaker... if not another 20 amp will be fine..