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How can I calculate power consumption of my PC in Watts, to prove my House owner (I live on rent), my PC doesn't consume much power? He blames me for huge power bills even though he uses a Fridge, A.C. etc and his son watches the TV all the time.

We both share one power meter so for bill we pay 50%-50% but he is saying I use PC all the time even at night - I keep it on for downloading.

I just want to calculate power consumption of my PC, then will calculate monthly expense of unit as per my city's per unit price for power.

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Someone give me the formula! – Ivo Flipse Apr 7 '10 at 6:33
P = VI ; E = Pt ... @ivo – quack quixote Apr 7 '10 at 18:08
up vote 16 down vote accepted

The definitive way would be to buy a Kill A Watt and plug your computer into it. That way you'll be measuring actual power usage instead of trying to estimate.

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+1: beat me to it. ;) – Ƭᴇcʜιᴇ007 Mar 23 '10 at 3:52
UK equivalent: Maplin L61AQ – Martin Mar 23 '10 at 9:20

I know an answer has already been accepted, but I just saw this:

Microsoft Research Releases "Joulemeter" Power Consumption Estimator for Windows

Joulemeter is a software based mechanism to measure the energy usage of virtual machines (VMs), servers, desktops, laptops, and even individual softwares running on a computer."

Joulemeter estimates the energy usage of a VM, computer, or software by measuring the hardware resources (CPU, disk, memory, screen etc) being used and converting the resource usage to actual power usage based on automatically learned realistic power models."

Download --> Joulemeter

To add to this: ESSaver is a free tool that shows the energyconsumption of the computer and even per process.

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You don't need to worry about all the internal bits of the machine to calculate worst case power usage. The wattage listed on the power supply is the maximum the machine can draw when under heavy load. It will typically draw far less. If you need a more accurate number then the Kill-a-Watt solution is recommended.

Remember that electricity is usually sold in units of kilowatt-hours. 1 kWh is the energy used by a 1000-watt widget operating for 1 hour.

So if your power supply is rated at 500 watts, and you leave it on for 24 hours, at the end of that time you've used, at most, 500 x 24 = 12000 watt-hours or 12 kWh.

Actual usage is probably far less, but without a meter it's very difficult to calculate.

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Nice edit @quack-quixote! – Chris Nava Apr 27 '10 at 17:11
"500 * 24 = 12000 watt-hours or 1.2 kWh" That would be 12 kWh. – ischeriad Aug 12 '10 at 9:53

As a quick rule-off thumb: if you want to know how much the power consumption of a device that is continuously on costs you per year, then you can just replace Watt with $ or EUR.

So, for example, if your constantly-on PC consumes 300 Watt --> it will cost you roughly 300 $ per year

(This is based on the assumption that 1 kWh costs 0.15 $ or 0.15 EUR.
So, 1 Watt continuously is 1*24*365 = 8.7 kWh/year, which costs 8.7 * 0.15 = 1.3 $
So, yes, to be correct, 300 Watt will cost you almost 400 $, but as a rule-of-thumb this simple approximation is good enough (for me).)

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Just get a multimeter and measure current on real time basis. Then use P=V*I. Assume the variation based on uses then calculate the waited avg.

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