I hate to be little johnny-one-note here (because I have a whole thread elsewhere griping about this issue) but I think I might have an insight that could be relevant. Modern hard drives have sophisticated power management, and even SSD will attempt to 'spin down' into a lower power state when not actually in use. Due to this bug, this can't happen because the system is CONSTANTLY WRITING TO THE HARD DRIVE.
In the thread I linked above I have a simple test to check for this .xsession-errors log spamming issue:
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Open the caja file manager on your home folder.
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Select "Show Hidden Files" from caja's "View" dropdown menu.
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Right-click on the .xsession-errors file and select the bottom "Properties"
selection from the resulting caja right-click menu. -
Note how quickly the file "Size" parameter is counting up in bytes/sec .
More than likely, if this is happening, all this HD activity is keeping your hard drive so busy that it can never enter it's low power state, and this could easily account for the small but significant extra power draw . . .