doug, et al.
I just stumbled accross this message linking back from Hal's message 10041.
VA is different from watts. My Internet Office model has an output of 500 VA, 280 watts.
The BC Personal is 500 VA, 315 watts.
Don't ask why the difference... that's what they say on the back.
Was jolted into thinking "Hey, I used to know that stuff!" After scratching the head a little, and getting out an old text book to verify, here is the difference:
Devices offer "impedence" to an AC power source that is usually not purely resistive. Capacitors and inductors both limit the current drawn from the power source, as would a resistor, but for these devices the current is 90 degrees out of phase with the applied voltage. They are referred to as "reactive" devices. Voltage leads current in an inductor (E=voltage, L=inductance I=current); current leads voltage in a capacitor (C=capacitance): "ELI the ICE man". For a pure resistor (R), the voltage and current are in phase.
The power consumed by a device, or circuit of devices, is (RMS voltage) x (RMS current) x (cosine of angle between current an voltage) or (VA)cos<theta>. cos<theta> is called the power factor, and can never be greater than one. A pure resistor has a power factor of 1; it converts all the electrical energy into heat/light ("dissipates" all the electrical energy). Purely reactive devices have a power factor of zero; they store electromagnetic energy, but don't convert any of it to another form.
Your UPS is not expecting to always encounter a purely resistive device, but if it does it can only deliver the maximum rated watts, a number less than VA always. It is also not expecting to encounter a purely reactive device, but if it does the most current it can deliver is VA/output voltage. Whether you get maximum current, or maximum power from the supply depends on the type of device you hook up to it. Of course generally, you don't fully load the supply so you don't max out either one, but the power consumed is usually somewhat less than VA/V, unless you are using it to run that heating pad you have under your feet.
Hope that helps.
Dan |
