Ohms law is relatively straight forward and can be used to calculate an accurate figure of wattage useage from any light system, and this would probably have been a better way to go. To calculate total wattage used from a ballast HID system you need to use the amperage figure not the bulb wattage figure. To show you how your figures are inacurate, OHMS Law is Volts x Amperes = Watts, so if we know two of those figures we can calculate the third.

The real wattage of a 400w HID system is Volts x Amperes or 120 x 3.8 = 456watts for an average 400w system others would be -

250w 120 x 2.5 = 300w
400w 120 x 3.8 = 456w
600w 120 x 5.7 = 684w
1000w 120 x 9.5 = 1140