Have I got it wrong

[Touche]

Hi everyone,

I have recently started Bitcoin mining as I bought an HD6970 for gaming and thought I could earn a few bucks back to help pay for the card. I am running GUIMiner on Windows 7 64 bit and get around 400MH/S. I have done plenty of research, and was thinking about maybe getting a dedicated rig with multiple PCI-E slots to try and earn some money. The sites I have looked at that estimate profit all seem to have one flaw - they don't take into account the increasing difficulty of mining. To this end, I decided to make a speadsheet that takes this into account. I was shocked at the results, since these seemed so much different to the results I had from the other sites. For example, my current rig looks like this :

http://i228.photobucket.com/albums/ee192/touche36/BitCoin/6970a.png
http://i228.photobucket.com/albums/ee192/touche36/BitCoin/6970b.png

a rig with 2*HD6990's looks like this :

http://i228.photobucket.com/albums/ee192/touche36/BitCoin/2x6990a.png
http://i228.photobucket.com/albums/ee192/touche36/BitCoin/2x6990b.png

and 4*HD5770's looks like this :

http://i228.photobucket.com/albums/ee192/touche36/BitCoin/4x5770a.png
http://i228.photobucket.com/albums/ee192/touche36/BitCoin/4x5770b.png

Have I done something wrong ? Have I missed some fundamental point ? I don't see the point in buying hardware, when you can only get such a small return on your investment (other than the fun and knowledge gained) :-)

If you want to download the spreadsheet (Excel) and check for mistakes, or put your own figures in, you can find it here : http://www.filefactory.com/file/cc3d4b5/n/Bitcoin_Caclulator.xls or here : http://www.megaupload.com/?d=L8CGMTC2

BTW I live in the UK, hence the electricity prices

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[Duffman]

you need to factor in psu efficiency.  if your psu is 80% efficient, you waste 25% more electricity that goes to heat (7.92KWh turns to 9.9KWh). if its 90% efficient, you waste only 11.1% more (7.92KWh turns to 8.8KWh), etc.

edit: nevermind; I see you're using a kill-a-watt measurement.  if you wanna use numbers from review sites, like Techpowerup, you would have to factor the efficiency.

[karoshi]

Next difficulty increase is coming soon (~1 day), mining will get very unprofitable soon for people living in countries with high electricity costs. Even with efficient machines (>2MHash/J).