Display resolution and power usage

[IntelUser2000]

The recent release of iPad with Retina Display and LTE showed something interesting. The quoted battery life numbers are 10 hours for WiFi and 9 hours on LTE. The WiFi battery life is actually similar to iPad 2, but the battery capacity increased significantly to 42.5WHr(from 25WHr) to achieve that.

That implies average power consumption of 4.3W for the new iPad versus 2.5W for the iPad 2. There are speculations that it might be due to the new A5X, but the contribution of power usage is far less. The real reason is due to the display.

According to the data I have:

Panel electronics power

1366x768: 1W
2560x1440: 2.2W

Backlight power

1366x768: 0.8W
2560x1440: 1W

That means quadrupling the resolution from 1366x768 to 2560x1440 increased total display power usage from 1.8W to 3.2W, a 1.4W increase. Only ~0.4W is due to the A5X SoC.


This might be why laptops have been stagnating in display resolutions. 1.4W extra power in light usage scenarios like browsing equals to 20% difference in battery life. Though manufacturers can use higher quality panels and more optimized components along with bigger batteries, its unlikely when the margins are already low and most vendors primarily compete on pricing. Higher resolution panels also cost more.

It's very possible newer Tablets with higher resolutions need extra power as well, but being a new market with higher margins(yet to be bottom of the barrel pricing) they can compensate it with bigger batteries. Also the ecosystem of Android based Tablets are more proprietary than PC Laptop market so more customization/optimizations are possible.

 

[Anteaus]

You may be right, but I always thought that it was a cost consideration. Wouldn't the backlight for a low resolution panel be the same as a high resolution panel, all else being equal (same brightness)? I have had high end laptops with high resolution displays and it certainly didn't seem like batter life was hurt. More to point, battery life seems to be directly related to the CPU/GPU and how they are used.

I think laptops stagnat in display resolution because people like cheap laptops and that is one way to cut costs. Another reason could be simply because many people don't have the eyes for high res. 1920X1080 is great for a 22-24" panel, but once you get into 12-14" laptop screens text/icons that small can really annoy some people.

Like you mentioned, tablets get away with high res because of there proprietary interfaces. Much more of the screen is taken up with interface when compared with a mouse based computer.

 

[IntelUser2000]

Higher end laptops usually end with discrete graphics cards and that significantly reduces the impact of the display. 1.5W isn't going to do anything when the laptop already uses 30+ watts.

However in thin and light form factors like Ultrabooks 1.5W is significant. That is made worse by the usage model being lighter than on higher end laptops.

Cost is an issue too. Both from the need for bigger battery and the expense from better display.

 

[Meaker10]

The major reason on laptops is cost.

As you add more pixels you have to increase the electronics behind the scenes more and more. This also has the impact of increased power consumption. The time you have with each pixel is reduced and the faster frequencies they work at too.

There is also just more pixels acting as more resistors on the circuit.

If it were a power issue we would see more 1600x900 screens or between. We get 1366x768 because they can make them in shedloads for next to nothing and because they use cheap components they are not even that power effecient.

My screen has a maximum power consumption of 11.5W:

http://www.scribd.com/barlow00/d/50785492-B156HW01-V-4

A similar 1366x768 is 6.5W:

http://www.avidia.com.tw/docs/product/B156XW01 V.2.pdf

That seems like a big difference until you realise that the 1080p panel is brighter and has a higher contrast ratio so you can probably half that max power to get the same brightness and still have it look better.

 

[kbp]

Whats the resolution on a Mac? If its better than most PC's I know they use "less" power than a PC display thus taking the power usage out of the picture here.

 

[Sunburn74]

Actually I believe apple's power savings come from the fact that the OS doesn't really have much in the way of background tasks. When you put windows on an apple laptop, they take huge battery life hits.