A coffee warmer maintains the drink 39 s temperature longer and prevents the coffee from getting cold before you have a chance to drink it sometimes called a mug warmer the small electric appliance plugs into a wall or usb port and uses a warming plate to maintain the temperature of the contents of your cup easy installation no driver required plug play on off switch controls warmer red led indicates when warmer is on and green led indicates when warmer is plugged into your usb keep beverage coffee or tea warm at 35 40 degrees c usb powered no batteries required note in picture shows four usb ports but in actual no any extra usb port direct plug in to usb port and enjoy color and design as follow pictures show size one size product dimensions 10 9 5 2cm usb cable length about 1 5 meters