I installed Coffee on a Windows 8 Tablet (Acer W510) but it seems do nothing.
I tried several settings (10, 20, 25, 50, 100 and 500 ko/s) and downloaded a big file (1,3 mo/s average) but windows did go to sleep always. I tried on firefox 19 and Internet Explorer 10.
In the Event log, i can see "The system is entering connected standby
Reason: Idle Timeout." a the time of standby mode.
even if Coffee is running.
I was very surprise that i didn't found alternative to Coffee for this extremely important function (and i looked for a while). Especially for the new Windows 8 tablets: You cannot start a download and go for multiple hours because you risk a autoshutdown of the PC.
Same with NAS upload/download.
Don't hesitate to ask me if i can help you!