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 Post subject: Measuring power consumption cheaply using a resistor inline?Posted: Sun Jan 03, 2010 3:29 pm

Joined: Wed Dec 30, 2009 9:13 am
Posts: 72
I see alot of people measuring power consumption with Kill-a-Watt, UPS readings, or using other devices. All along, I've been measuring my power consumption using a very cheap and simple technique.

I modified a power cable with a 1 ohm 10 watt resistor inline of the neutral wire (not the live wire, because that would be dangerous). Then by measuring the voltage drop over the resistor, you can calculate the current that is flowing and then the power consumed by whatever is plugged into the modified power cable.

Danger aside, I am just wondering how accurate this method would be compared to the other methods especially with power factor taken into account? I'll admit I wasnt paying much attention in physics class during the power factor section =( Are there other things to take into consideration? I measured the power consumption of various lamps and this method seems accurate enough.

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 Post subject: Posted: Sun Jan 03, 2010 9:40 pm

Joined: Wed Nov 11, 2009 2:31 am
Posts: 51
My \$20 power meter serves me well, and I can find out how many watts other electronics consume too.

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 Post subject: Posted: Mon Jan 04, 2010 12:18 pm

Joined: Fri Jan 20, 2006 10:12 am
Posts: 563
Location: UK
Power factor can make a very big difference. For instance, a PSU with no power factor correction might have a power factor of 0.6, which means that if you were to see an apparent power consumption of 100W, then the actual power would only be 0.6 x 100 = 60W.

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 Post subject: Posted: Mon Jan 04, 2010 12:46 pm

Joined: Wed Dec 30, 2009 9:13 am
Posts: 72
I have an Earthwatts 380W, which is supposed to have APFC, so Im assuming power factor is negligible?

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 Post subject: Posted: Mon Jan 04, 2010 1:38 pm

Joined: Fri Jan 20, 2006 10:12 am
Posts: 563
Location: UK
APFC should keep the power factor well above 0.9 for normal loads (it will be much less in standby). You could try looking up a measured value for your particular PSU (the 80 Plus website is a good resource for that sort of thing).

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