A USB cup warmer is a small, portable device that uses a USB connection to maintain the temperature of a beverage, typically keeping it warm rather than heating it from cold. These warmers typically plug into a computer's USB port and are designed to keep drinks at a comfortable drinking temperature, often between 104°F and 140°F (40°C and 60°C). 
Key Specifications:
Power Source: USB (usually 5V/2A). 
Wattage: Typically 10 watts. 
Temperature Range: Often maintains drinks within a range of 104°F to 140°F (40°C to 60°C). Some models may offer adjustable temperature settings. 
Size: Dimensions vary, but many are compact and designed for desk use. 
Material: Common materials include aluminum alloy, tempered glass, and plastic. 
Cord Length: The cord length can vary, but 29 inches (74 cm) is mentioned as an example. 
Heating Mechanism: Some warmers utilize a heating plate or a heating coil, and some may include features like auto shut-off. 
Other Features: Some models may have multiple temperature settings, timers, or gravity-sensing switches.