There’s no universal rule about what connections portable monitors have to offer. From one to the next, you can find any combination of ports. Some portable monitors rely on just a single USB-C connection for power and video, and others may need auxiliary power. Many of the best options include a USB-C port to handle all your needs, so you only need one cable. Most also include an HDMI or mini HDMI port, which provides broader compatibility. Newer laptops can handle display output over USB, while game consoles, older laptops and desktops might not, so having any kind of HDMI port is useful.
### Power
Some portable monitors can draw all their power from the same USB-C cable they get their video signal. Bigger, brighter and higher-resolution monitors typically need more power, and this can be too much for USB ports. In these cases, the monitor may have a second port for auxiliary power. Unfortunately, the matter of powering portable monitors is incredibly variable. Some laptop USB ports _can_ supply enough power for a portable monitor, but another port on the laptop might not. Some monitors I’ve tested will work fine with a single USB connection to a laptop but then turn off if the brightness is set too high. Given the variability, it’s important to check each monitor's specific power requirements and try to ascertain whether your computer has a port to meet the requirement. If you’re particularly concerned about power on the go and using fewer cable connections, some portable monitors include built-in batteries.
### Size
Size is an important consideration for a portable monitor. You want to maximize the additional screen real estate you’re getting on the go, but you also want to avoid ending up with a monitor that’s so big that it limits mobility or uses too much desk space. You’ll generally find a listed size with each monitor that indicates the diagonal size of the display area only. So, to make sure the monitor you’re looking at will actually work for you, be sure to also check the physical dimensions of the monitor.
### Color
Just like size and resolution, color gamut is an important consideration. Put simply, it’s a measure of the range of colors a monitor can display, and it’s presented as a percentage of a specific color space. Lower ratings will mean a less vibrant, more drab monitor. Higher ratings mean more lifelike colors. If you’re just doing office work and want extra space for spreadsheets, just about any monitor will do, regardless of color gamut. A 95% or more rating for the sRGB color space is good for entertainment and general web work. Monitors that cover 95% or more of the DCI-P3 color space can display a wider range of colors, making photos, videos and games more enjoyable, but these will generally be more expensive options. For professional use, you'll want to check for a measurement of color accuracy, too; a value of Delta E≦2 is optimal. Most portable displays are not color-calibrated out of the box and can look significantly better after calibration with [a colorimeter](https://www.amazon.com/Calibrite-Display-123-Colorimeter-Calibration/dp/B0CW27QFWN). Short of that, you can use tools in MacOS and Windows to improve image quality manually.