Just another quick note - most cheap HPS security lights (that I've seen) do not come with a capacitor which more than doubles the operating amperage and is quite misleading.
My cheapo 70 watt HPS without a capacitor draws 1.6 amps on 120 volts. That's 192 watts, not 70!
If your 2 150 watt HPS ballasts don't have capacitors, you are most likely going to draw around 6.4 amps or 768 watts worth of consumption.
With capacitors, 2 150 watt ballasts will pull around 3 amps or 390 watts.
Don't get the ignitor and the capacitor confused. The ignitor is required for HPS operation, the capacitor is optional on smaller wattages.