Energy used by electronic ballast

I have been searching high and low for this answer, I have a 400 watt and 250 watt HID light set up in my grow tent, 400 is HPS and 250 is MH and I know the watts drawn by both is 650 watts for just the lights alone, what I would like to know is ...how much energy is used by just the ballasts them selves? I'm taking a ballpark guess at 50 watts each...anyone know? please help me on this, as I doubt the rated watts of the bulb take into account the amount of energy used by the ballast...anyone? please help me.

Yeah, a little of the energy is lost to heat and magnetic field generation, they're not 100% efficient. However, electronic ballasts are more efficient than magnetic ones. Magnetic ballasts are around 80%-85% efficient while electronic ballasts are often 90%+ efficient. You can do the math.

OK, I'll do the math. Let's be conservative and say your ballast is on the low end, 90% efficient. That means it's going to use 10% more power than the bulb is rated for. Therefore, your 400 watt ballast will actually consume around 440 watts and your 250 will actually use around 275 watts.

Thank you for the reply, I knew I wasn't too far off, but knowing for sure sets my mind at ease...just wanted to figure out just how much juice I was using so I could figure cost more accurately, I'm funny like that.