(more nitpicking) But don't you have a tiered rate? -- the first XXX KWH at 0.14, the next YYY at 0.17, etc.
As for using 68w as the power difference, that was based on the measurements when the computers were doing nothing, its worse at full load.
When I used the standby mode at work, the IT guy complained that the backup program could not wake up the PC for backups.
...unless of course they're running file-sharing networks
LOL, yea, I sneak around the house turning off the teenagers' PCs every night.
Petz