Eight years ago I, too, purchased a "Green Plug" by Green Technologies
after seeing the same side-by-side "power-saving" display at my local
Home Depot as described by MMDer Ray Finch.  It's physically the size
of a pack of cigarettes.

For any "power saving" device to be cost-effective it must be used on
the most energy-hungry continuously operated devices in the home,
like the refrigerator.  Since purchasing it eight years ago it's been
continuously providing the power for my refrigerator without incident.

With the device, when you open the door the interior light dims
slightly but noticeably after just a moment -- presumably that's the
device working.  When the compressor starts it sounds normal, but then
after a second or so you can hear the RPMs artificially drop slightly,
again the device supposedly doing its job.

The directions said that refrigerant compressors would run more 'cool',
so it may be modifying the power-factor (a trick purposefully done in
heavy industry with many motors running) after providing unaltered
current for start-ups -- so a motor may fail to start if the device
fails in it's delay period.

A few years ago I asked a Home Depot employee why these devices were
not longer sold, and he said "because they don't work".  But I still
use mine today, in case they do.  And I paid for it anyway.

Applying the device to a player piano, I'm not sure any of the above
would be welcomed behavior.  Nor would you save much power on a device
that's run only minutes a day.

Karl Ellison