Originally posted by Dr_simon
Lordathestrings, this is the transformer I have:

http://traveloasis.com/transformers9.html

I’m sucking 785.8W (allowing 100W for my PC -with no monitor) through an ART Power distribution unit rated at 1800W @ 15A

Please let me know if you can see any problems!

I found some stuff on the manufacturer's website. The FAQ button leads to some quasi-helpful answers, but they don't address the frequency/efficiency issue. One is left to assume that the spec is for the worst-case condition, an unwise assumption.

The suggested Wattage calculation ignores an inconvenient detail called the Power Factor. Multiplying the Voltage x Current only gives an accurate value for the power consumption if the voltage and current are [u]sinusoidal, and in phase[/u]. When power is drawn through a transformer, some phase shift is introduced between the voltage and the current. The amount of this shift is the Power Factor.

The power supplies of most consumer electronics use a capacitor to store DC energy (rectified AC), to smooth out the pulses of the AC line voltage. This means that there is very little current drawn until the voltage at the rectifier output exceeds the voltage stored on the capacitor. Then, there is a sudden increase in the current, as the rectifier 'tops up' the storage capacitor. The resulting AC current waveform has sharp spikes on it. This is definitely not a sinusoidal waveform.

The bottom line? The power calculations based on nameplate information is only an approximaton, and may be off by as much as 20%. The reduced efficiency at 50 Hz could mean that your present setup is more than one transformer can handle. In industrial applications, where reliability is critical, it is common to select components with a 100% over-rating. That is, with twice the capacity of your expected 'normal' load.

I suggest asking AED some pointy questons about how, and under what conditions, their ratings are calculated, and how much power you can expect to draw from the output when the unit is fed from 50 Hz British AC mains.