[rossb]

I am reaching back into my memory a bit but I think it goes something like this. The power factor is not something that you can control, it is controlled by your electric utility. The AC electricity that arrives at your house has a sinusoidal waveform. A very simple analogy would be to think it of as two phasors that roughly are the voltage and the current. If these waveforms arrive at your house in phase then the power factor is 1. If they arrive out of phase then the resultant power that you get is reduced. Altought the mathmatical calculations are not trivial if they are 45 degrees out of phase the resultant power would be about 70 percent of normal If they are 180 degrees the resultant power would be 0. Typical power factors in residential areas are in the region of .8 to .9.. So when you select a ballast the power factor consideration relates to the power you receive. If you have a high power factor (.9) you select a high power factor ballast. If you have a power factor of .8 you select a ballast that operates at that PF. Your lighting supply or local utility can tell you what your power factor is.


So if you have a 1000 W light, and assuming .8 power factor, you have to supply 1250 Watts. Also the ballasts are not 100% efficient so I expect to get 1000 watts of light it takes about 1400 watts.

Basically power ~= voltage*curent*powerfactor