Why does the draw form a 400w MH calculate to 455w?

Question

If i have a 400w Metal Halide fixture why does the examples I am given around 455w? And a flourescent 4 - T5HO54W Fixture reads somewhere in the 230 range? Way above the simple calculation of addition...How do I find the draw for 1000w? And is the a calculation to figure out this info? I need it for a energy summary.
tHX!

Answer 1

Power = Volts X Amps so depending what voltage number you are using you will see some variation. Try your calculation at 110V, and 120V and you will see the difference.

Answer 2

I'll bet your have losses in the MH lamp like those in a fluorescent ballast. An 80 watt fluorescent fixture will draw about 100 watts.