Beware of power supply efficiency when calculating.

A particularly poor case that illustrates this point is me testing a celeron j1900 where the Kill-a-Watt clone shows it using 17.5watts but the voltage reading x amps reading reported is in the high 20's for an efficiency figure of 65% and an actual use of something like 28.5v/a

If you want something frugal dedicated to crunching this celeron is probably it. The newegg price of board and ram was $89 when I bought this but in UK I had to pay just over £80. Annual running cost may be as low as 162Kw/h or as high as 250Kw/h depending on the psu solution as shown above.

On 10th of May I finalised the bios settings etc for the one I am testing on FAHV and Linux Mint. You can find the results here:

http://stats.free-dc.org/stats.php?page=host&proj=bwcg&hostid=3302786

Looks like 1900 Boinc ppd and 3d 18h approx per day on this project.

The xeon D-1540 will be cheaper to run (per core) and give better performance in terms of ppd (per core) but at over £700 for just the board and cpu it is in another class entirely.

Damn.... I could be taking this thread off topic....where I come from that is normal

Thanks
I am running Linux Mint 17.1 and Conky procesor temp on desktop. The procesor temp. oscillates between 50and 57C.
Sorry for the offtopic.

Power companies only charge you for active power, not reactive. I*V in AC will give you apparent power (sum of both), although I don't know what the power factor of a computer is (not good I'd assume since it has a rectifier and other non-linear loads...)

With that said, most PSUs are most efficient close to their nominal load (100%).

That celeron looks pretty efficient. The latest CPUs from intel are very powerful per watt!

That has historically been the case, but the newer 80 Plus high-efficiency power supplies are required to maintain a fairly high efficiency over a wider range now. I think 20% to 80% load is quite acceptable.
http://en.wikipedia.org/wiki/80_Plus

Sorry to keep this thread OT, but one other factor to consider when thinking about swapping a home pc for cloud computing is heat. When you crunch at home the power that goes in the plug mainly exits the back of the computer case as hot air. In hot climates this will add cost for cooling the air through fans/aircon, but it cooler climes this will save you money on your domestic heating bills.

Site your PC in a corner of the living room and let it provide a bit of background heat and you could possibly turn off your central heating for longer each year.