Okay, I know the voltage (120VAC), and the amperage (0.17 amps). So how much power is it using? (this is my new 24/7 server box)

The simple equation is Watts = Volts * Amps, but with alternating current (A/C), this is not exactly correct. I know that much, but I don't know how to calculate it correctly.

Could one of you please educate me on this?

Thanks

Ugh.. there may be no simple answer, as shown here.

Cheers

You cannot simply calculate the power unless you know the power factor of the circuit. The simple equation works for DC or when the load is a perfect resistance (ie no inductance or capacitance). When inductance or capacitance is present (ie nearly always) the current phase either leads or lags the voltage phase such that the instantaneous power at any point on the voltage cycle is always less than it would be if the current was in phase (perfect resistance). The power factor is always <=1

Briefly put;

VA = V * A. This is the maximum theorectical power for a pure resistance.

Power = Power Factor * VA. This is the acutal disippated power.

A reasonable non-mathematical explanation can be found at http://www.microconsultants.com/tips/pwrfact/pfarticl.htm

In this particular case (a PC), almost all the power consumption is resistive. If you measure the output current of the PSU, that's DC and thus is all actual consumed power. It constitutes an underestimate of total power consumption (it doesn't include losses in the PSU, which gets warm and so is consuming power), but, if your PSU is reasonably efficient (which they are these days), it's probably close enough to the overestimate produced by 120V*0.17A to give you a ballpark electricity bill figure.

Peter

The difference between the measured D/C VA and the A/C VA is about 2 VA (~watts), which accounts roughly for what the 12V power brick heats up from. I had another power brick that used more, but have switched them after discovering the new one seems more efficient.

Thanks

So Mark, what are you using that's only drawing 20VA? That's pretty low power for a server type box - I assume it's a firewall of some description?

What Peter said about PCs being generally fairly resistive (i.e. close to unity power factor) is true. I just measured a normal P3 1GHz PC here (the company I work for designs and manufactures solid state energy meters so I have some background and the equipment to easily test) and the power factor was around 0.97 (12 degrees phase angle). So you're looking at 120 x 0.17 x cos 12 = ~20W. For all intents and purposes the power factor makes stuff all difference in this case.

The other way to work is to go backwards from the DC output and assume roughly 75-80% efficiency in the PSU. i.e PSU output / 0.8. Does that line up?

I just wish I could get my MythTV box down that low. It currently draws around 120-130W. XP2400 and 3 7200rpm drives doesn't help of course...

So Mark, what are you using that's only drawing 20VA? That's pretty low power for a server type box - I assume it's a firewall of some description?

Ahh OK. Found your other thread about the Epia mobo.