Our Panel PC will be outdoors in direct sunlight. How many nits does the display need to be readable?

Outdoors in sunlight, choose a display of 1000 nits or more. A typical indoor screen is only about 250–400 nits and is barely readable in sun. Brightness is not everything. Three other factors decide readability: 1. Optical bonding: filling the air gap between the touch glass and the LCD panel, which sharply reduces reflections. At the same 1000 nits, a bonded screen is clearly easier to read. 2. Anti-glare and anti-reflective coatings on the glass. 3. Orientation: avoid facing the screen towards the afternoon sun, and add a hood if possible. A brighter screen also produces more heat and adds to the machine's temperature in the sun. Choose a machine with an ambient light sensor to dim at night, which reduces heat and extends backlight life. Also check the operating temperature of both the machine and the LCD panel: a panel in direct sun can get much hotter than the surrounding air.