Thinking about resolution in megapixels helps illustrate the scale of the difference. The 11-inch MacBook Air is 1MP (1 million pixels). The 13-inch model is 1.3MP, and the 1080p XPS 13 is 2MP. The high-res Dell XPS 13 is...wait for it: 5.7MP.
More pixels use more power
As we all know, an increase in resolution can impact battery life. We've seen it in phones and laptops. Pixel density is nice, but it isn't free.
I queried Dell's laptop panel expert, who summarized it this way: The circuitry to drive the panel eats more power. A 1080p panel might consume 0.9 watts, while a 4K panel controller might use 1.3 watts to 1.5 watts. That's significant when you consider that the monitor generally consumes the most power on a laptop today.
Then there's also the power needed to light higher-dpi panels. Because the pixel pitch is much tighter, it blocks more light. That means it takes more light to get to the same brightness level of a lower-resolution screen.
One way to get around that is to use different screen technology. IGZO panels, for example, let more light through than a traditional high-res IPS panel, so you don't have to burn as much power to hit the same lighting levels.
It's a trick Lenovo/NEC is taking with its LaVie to cut the weight to 1.76 lbs. By using an IGZO panel, Lenovo can reduce the battery size while maintaining acceptable brightness and run time.
In my XPS 13 review, I used BAPCo's MobileMark 2014 test. It's the updated version of the industry-standard MobileMark 2012. It uses off-the-shelf and popular applications, and it runs them through various tasks at normal speeds.
MobileMark is unlike most rundown tests in that it acknowledges typical users' tendencies to take breaks or zone out like Office Space's Peter Gibbons. MobileMark replicates this by allowing long pauses and letting the screen go to black.
The above test shows the battery life performance of the XPS 13 with QHD+ screen against the larger and older Lenovo X1 Carbon 2014. Despite its higher-resolution screen, that's decent battery life in the Dell. The X1 Carbon is a different beast: It has a 14-inch screen and a 45-watt-hour battery. Keep in mind, that's with the Wi-Fi hot (but connected to a router that goes nowhere), which is required by MobileMark 2014.
What about the MacBook Air?
There is no MobileMark on the Mac that runs in OS X. Yes, I could install Windows 8.1 and dual-boot, but I'm not sure that accomplishes much. I'm also not sure how much actual optimization Apple does for Windows. I suspect the company would rather have its users boot into OS X and run Windows apps in Parallels.
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