# Calculating electrical power/cost usage of my computer

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June 14, 2007 6:57:24 PM

Just had my bi monthly electric bill through and it appears to have doubled in usage and I was wondering whether my computer was having anything to do with it.
How do I calculate the usage(cost) of my computer in kWh?
I have the following info:
Computer uses 375Watts according to Outervision Extreme power calculator + unknown Wattage of a Samsung 19" LCD monitor. The electric costs 0.1592Euros per kWh and if we say 15hrs of use a day with 284 kWh total electric used in two months.

I have no idea if the above is enough info but if someone can give me some pointers that would be great.
Cheers.
June 14, 2007 8:07:15 PM

Quote:
Computer uses 375Watts according to Outervision Extreme power calculator

...any power consumption value from a web site will be a bit approximate, but provided that you take that as an estimate, that will be fine... Alternatively, over here (£-land) you can buy an energy monitor that actually monitors electricity usage for about £30 (and there are some more expensive ones, too).

Quote:
+ unknown Wattage of a Samsung 19" LCD monitor

...at least you don't have a big CRT, they do suck up power, even on standby... Well with a CRT at least there would have been an easy cost saving suggestion (hit the power button) and a less easy one (buy a new screen). With an LCD or a TFT the consumption will be a lot lower and the power will go down reasonably when the screen is blanked.

Not quite sure what you are struggling with in the calculation:

375 Watts is 0.375 kW (by definition).

15 hours (big number for actual use; is it use or is some of the time computer idle time?) of use makes that 5.625 kWH per day. 30 days per month? 168.75 kWH per month. 20 days? 112.5 kWH per month.

(Neither of those is your figure of 284 kWH: that would need 50 days over the two months - 25 days per month.) 25 days per month of usage doesn't sound implausible.

284 kWH at 0.1592 per kWH is 45.21 (Euros). If this is a significant part of the bill, then yes, the computer use could be a big part of your bill. If you have a lot of your computer time with the computer idling, you should ensure that any power saving options are enabled and while hitting the power button on a TFT doesn't save as much money as it does on a CRT, you should still use it.

Equally, an efficient power supply saves you money in the long term over an inefficient one (and probably isn't taken into account by simple web estimation tools - they probably don't know the efficiency of your power supply). Unfortunately, while most suppliers want to tell you that their power supply is efficient, that isn't backed up by reliable numbers that you could use.

(I always have difficulties with the fact that the cheaper power supplies are, how can I put this, optimistically rated, but I've got used to putting in a 'correction factor' to allow for this: however, I can't really do the same thing about a company that claims their power supply is '80 percent efficient'. Over what range of powers (i.e., at the power level I actually use?)? And if believe that the manufacturer is 'optimistic' about its ratings, do I now apply a correction factor to that 80%? If I use my 1.5 factor and come up with an efficiency of 53.3% I won't buy the PSU, because it will be hot, noisy and eat power. In other words, I will always buy a supply from a manufacturer whose numbers I can trust over one that I can't)
June 16, 2007 1:57:42 PM

Thanks, Nop.
Of course, the blindingly obvious, 375Watts is 0.375kW.
To answer your questions it is running a lot of that time on idle and in actual fact I did the online calculation as if it were running at 85% which I then changed to a calcualtion of 60%, which is the lowest % it could calculate, and came out at about 192Watts.
The amount I run it for is variable, but it is usually on all day and evening and, quite often, on overnight for downloading.
I have an Enermax Liberty 500Watt.
I'm probably going to buy a Zalman ZM-MFC2 fan controller which has a built-in power consumption meter or I might by an in-line meter.
Thanks again.
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June 16, 2007 3:57:43 PM

Quote:
Just had my bi monthly electric bill through and it appears to have doubled in usage and I was wondering whether my computer was having anything to do with it.
How do I calculate the usage(cost) of my computer in kWh?
I have the following info:
Computer uses 375Watts according to Outervision Extreme power calculator + unknown Wattage of a Samsung 19" LCD monitor. The electric costs 0.1592Euros per kWh and if we say 15hrs of use a day with 284 kWh total electric used in two months.

I have no idea if the above is enough info but if someone can give me some pointers that would be great.
Cheers.

Ok. Let's take the maxium power your machine could use 500watts/hr.

Next we calculate the kwhr equivalent so 500/1000 = .5kwhr for your computer.

Moving on:
.5kw/hr * 24 hours a day = 12kwhrs
12kwhrs/day * 62 days = 744 kilowatt hours
744 kilowatt hours * 0.1592 Euro = 118.44 Euro increase in energy cost

If you want to find a ratio after you figure out exactly how much power your system is using, do it like this for example:

375Watts/500Watts * 118.44 = 88.83 Euro

This is the equivalent cost you would have for the machine for two months if it was running at 375Watts 24/7.

Hope this helps, let me know.
June 16, 2007 4:13:42 PM

Got my energy monitor for £11.99 off eBay.

It gets a lot of use, as I do solar powered cinema, and all the hardware has to be checked for idle/load power consumption, and tweaked for minimum usage while running.

G3 ibook for playing films is about 12w, but the projector pulls 220w and the surround sound amp about 110w. I also do VJing, and adding the stuff for that hits 800w.
June 16, 2007 5:45:21 PM

Cheers all.
Will do some messing around with the calculations and my revised numbers
!