The screen will be dimmer when running on batteries alone, but bright when you hit the button on top, unless you have the USB cable connected in which case the screen will be really bright. The display will only be backlit continuously when connected to a power source with the included USB cable or a DC adaptor (not included). 3 x AAA batteries (not included) are required to retain time and alarm settings Features & Specifications:– 12 or 24-hour format– LED screen– Alarm and snooze function– Temperature display 0-50C– Night screen brightness setting– USB power cable included – Size: HWD 9 x 17.5 x 8.5 cm / 3.5 x 6.9 x 3.3in– Materials: Acrylic case, Black LED?screen– Colour: White/Black– Movement: Digital– Power: 1x USB cable included, 3 x high-quality branded AAA batteries (not included)