How Much Is This Pc Costing In Electricity?, Calculate it

The easiest way to determine your average price is to look at your monthly electricity bill, and divide the total charge by the usage (kWh). 

For example, if your monthly charge is 37.88 and your usage for the month is 289 kWh, then your average price per kWh is 37.88/289 = .13 cents/kWh or the Rate. This happens to be the Rate in NE N.J.
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If you leave the PC on 365, 24/7 calculate like this

Cost ( annual) = Watts / 1000 x 8760 (hours in a year) x Rate. Use the wattage of the power supply

Example: 
Cost = 400/1000 x 8760 x .13
= .4 x 8760 x .13
= 3504 x .13
= 455.52 per month 37.96
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Reduce the number of hours to 40 per week on the rest off.

Cost = .4 x 2080 x .13
= 832 x .13
= 108.16 per month 9.01
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Substitute your Watts and Rate and your hours 
Not 1000% accurate but quite close.