How to calculate energy consumption?


macrumors regular
Original poster
Jun 7, 2008
Hi everyone, i want to calculate energy consumption per month of a theoretical scenario...

Say I have a computer with a 750W power supply and I leave it on all day for a month. Judging by the fact that the computer wont be using its full power during idle time (meaning CPU usage is well below 5%), how can I make an accurate calculation of the consumption per month?

I think the electric bills in NY are about 7 cents per kilowatt hours...

I came up with 5 dollars a month but I know that's wrong lol...

Any help would be greatly appreciated!


macrumors G3
Dec 29, 2007
The easy way to do it is to get a meter that'll measure the current draw by the device. Otherwise, it's just going to depend on what you have installed in your PC. Most of my computers, even the ones with large power supplies, seldom draw over 100W total under a load. Most average 50-60W.

But even then it can be hard to calculate. My electric company (Socal Edison) have so many tiers of consumption, peak vs. non-peak, and so on - it's hard for me to know exactly what I'm paying per kilowatt hour anyway.

I'd just leave the thing on for a month and see what the difference in your bill is.