Square Peg wrote:
Most coffee mug warmers are under-powered. One reviewer on Amazon said
that they were restricted to about 24 watts by law.

This one

http://www.amazon.com/Bunn-BCW-Warme...1796737&sr=8-3

is made for warming the whole coffee pot. It says it's 120V/15amp.
Several of the reviews say that they use it for a single cup, but it
keeps it too hot. It only has on/off -- no temperature control.

Can I put it on a 300 watt dimmer switch like this?

http://www.amazon.com/Lutron-Electro...1799800&sr=8-2

If not, is there a way to ramp the the power down as an external
thermostat?
Yes. It's a resistive load, just like a light bulb. I find it
difficult to believe that 15 amps is correct, that would be 1800 watts
which will do far more than warm a coffee pot.

If you use the Lutron dimmer, make sure that the warmer is under 300
watts, or get one a bit more robust.