It’s typically the case, but it’s not quite that simple.The Hardware Reason: LCDIn the case of an LCD display, all of your light and nearly all of your power consumption is from the LED backlight • the LCD itself only uses a tiny bit of power. And you might imagine that your LED doesn’t have to change relative to the LCD, which is also true.But here’s the thing• as you can see in the image, there’s a pixel/subpixel pitch • the size of a pixel or subpixel cell • and there’s the pixel/subpixel fill percentage • the size of the LCD pixel that’s actually transmitting light. It tends to be the case that as the LCD grows in density, the overall pixel fill rate is reduced a bit. This defines the effective transmittance of the display, how much light actually gets through. And so, to get the same relative brightness on a higher resolution screen, you need a bright backlight.So it’s reasonable to expect that a higher resolution LCD display will take a bit more power, because of the need for a brighter backlight, than a lower resolution display. However, it’s not always true, because technology advances in the direction of lower power as well. One of the reasons for the size of the pixels vs pixel cells is the surrounding electronics. When companies started using IGZO (Indium-Gallium-Zinc-Oxide) transistors rather than amorphous silicon, they could shrink the transistor size and still deliver the same power. So IGZO displays got higher density without needing more light, at least over one generation. Going to quantum dot LCDs lower the power needed to deliver the same brightness of LED. So in comparing an older generation device to a newer generation, you can’t necessarily expect a higher resolution display to use more power.For any display, there’s also the issue of display refresh. And yes, it certainly would take the hardware more power, all things being equal, to refresh 24 million or 12 million subpixels than 6 million subpixels. But all is not necessarily equal. New generations of display hardware move to smaller chip geometries and lower voltages, so they don’t necessarily use more power, generation to generation. Within a single generation, the larger display’s refresh probably does use more power, but this is still fairly small compared with the issue of making light.The Hardware Reason: OLEDOrganic Light-Emitting Diode displays are different, in that the pixels/subpixels actually do emit light. So going, say, from a 1080p to a 1440p display, you have twice as many OLEDs, so the expectation might well be a display that uses twice the power.However, that’s never actually the case. For one, when going to higher density pixels, those LEDs are made smaller, yet, but specifically, lower power. That’s not just for marketing the higher density display, it’s actually a critical design factor. Because OLEDs actually do emit light, they’re also emitting a bit of energy in heat rather than light. Too much heat in an OLED panel and the Organic LEDs gradually start to fail. In fact, in some of the very early and hyper-expensive OLED televisions, this was a pronounced effect in 5–6 years. On my Galaxy Nexus, the OLED display was definitely starting to show its age after only three years.Some of Samsung’s AMOLED displays also use a “diamond pattern” in their OLED. If you’re a photographer, you may have seen this before • this is exactly the same kind of arrangement of photo sensors and filters on nearly all photo image sensors, called the Bayer pattern, after Bryce Bayer of Kodak, its inventor. In this, there’s a green subpixel for every display pixel, but red and blue are interpolated between pixels. This allows a display to use fewer actual OLEDs. It was first developed in an effort to keep OLED displays cooler and, thus, longer-lasting, but it’s also a power savings. And not something terribly noticeable on higher resolution phone displays.However they manage it, higher density OLED displays absolutely have to lower their per-pixel/subpixel power consumption. But it’s a goal of all OLED displays to reduce power for longer life. So there’s no guarantee of any specific relationship between power consumption and density. For example, the Samsung Galaxy Note 4 AMOLED display moved to 1440p, versus the 1080p display in the Galaxy Note 3. And yet, the display uses 14% less power.Another Power Issues• Maybe?I was reminded about other issues with OLED displays. The well-know issue is, of course, that OLEDs create their own light. So a totally black screen is using almost no power, and a totally white screen is fully maxxed out. You can dim your overall display brightness to save power, but that runs into another another issue with OLED that’s not well known. But this was pretty visible, at least back on my old Galaxy Nexus.On an LCD display, your screen brightness is, of course, controlled by a PWM (pulse-width modulation) controller driving the backlight LEDs. That sets the brightest white you’re going to see, and the LCDs control all the dimming. It’s that cross polarization that sets the blackest black, which is why a typical LCD only manages around 1000:1 contrast ratio.The LEDs are pulse modulated because they don’t really work well with a dimmer, like you might think of for an incandescent light. The LED itself is a based on electrons jumping a band gap and releasing light. It’s functionally a voltage source• if you increase voltage against the LED’s characterics, it fights the LEDs natural voltage level and burns out quickly. If you drop below the characteristic voltage, electrons can’t jump the band gap, and so, no light. But LEDs turn on and off wicked fast, so we pulse an LED really fast to control the brightness.That’s one reason an OLED display can have a 1,000,000:1 contrast ratio. The LED is off when you want pure black and fully on (well, three subpixel LEDs) when you want white. But you can slice that pretty thinly in-between, based on the pulse-width sent to each LED. But here’s the thing: you have to use that pulse modulation to control the overall screen brightness as well as the 256 or 1024 or whatever levels you want for the actual display content.So when I set the overall brightness too low on the Galaxy Nexus, the display essentially broke down• different rows of LEDs showed what was essentially patterned noise. The problem seemed to be that I was hitting the minimum on-time, and it wasn’t necessarily the same, row to row. You’d never see that with a brighter display, but it does set a lower limit on your power saving capability. It’s quite possible they’re getting faster OLEDs with the smaller subpixels on today’s displays, or they put a dimmer limit to prevent that ugly display. Of course, setting a dark theme will also save power on an OLED phone.The Software ReasonYou’ll find a number of modern phones • I know Samsung offers this • allow you set a lower screen resolution for some power savings. Obviously, they’re not actually shrinking the physical display resolution, nor are they only lighting up every fourth or sixteenth pixel. Samsung of course uses OLEDs in their premiere phones, so they could do that, it would save power, but it would deliver an unusable display.What they’re doing is restricting all of the operating system and GPU drawing to a smaller area, and using lower power rescaling, or even hardware pixel duplication support, to fill out the display. It certainly takes more CPU and GPU power to fill a 2160p or 1440p screen than a 1080p or 720p screen, so that’s what they’re saving on in these modes.