Watt (W) measure the rates of power in a specific point in time and watt-hour (Wh) measure the amount of energy consumed over and hour. If an UPS is rated for 100W and your device uses more than 100W at any point in time it will go in overload protect mode because it exceeded 100W. The Wh rating indicates how long your UPS can keep on powering a device. As an example, if your UPS is rated 75Wh, it will power a device that requires constantly 75W for one hour (75Wh / 75W). An a device that requires 7.5W constantly should be powered for 7.5 hours (75Wh / 7.5W)