Sunday, July 25, 2010

Does screen brightness significantly affect battery life?

Many laptops will automatically dim the screen when running on battery power in order to conserve energy. But does this significantly increase the battery life? How much power is actually saved when the screen's brightness is turned down? My guess has always been that the energy savings are negligible, that a darkened screen might give the laptop 10 more minutes of battery life, nothing more.

To discover the answer, I ran a small experiment. I ran a script which polled the battery data on my netbook every 5 seconds. I monitored this data for 2 minutes with the screen on its brightest setting and 2 minutes with the screen on its darkest setting. I disabled my netbook's wireless network card and muted the speakers to avoid any interference in the data. The results were marginally better than I expected:

Avg. mABat. life
(4200 mAh battery)
Brightest1120.83h 42m
Darkest1018.64h 7m

If kept on its darkest screen setting, my netbook battery would last about 25 minutes longer. If the brightness were to be set somewhere in between the two extremes to a level where the screen would actually be readable, the energy savings would probably be about half that--12.5 minutes.

The battery data was collected from "/proc/acpi/battery/BAT0/state". The script used to poll the data was written in PHP and is shown below:

 $state = file_get_contents('/proc/acpi/battery/BAT0/state');

 preg_match('/present rate:\\s+(\\d+)/', $state, $matches);
 $presentRate = $matches[1];

 echo date('M d G:i:s'), ' ', $presentRate, "\n";



Anonymous said...

I think you need to get out more.

mangst said...

This was just something that has always bothered me. Ok, the screen dims so your battery can last longer. That's great. But now, you can hardly see the screen, which is why you are in front of your computer in the first place! Is making the screen more difficult to see worth the extra battery life that it gives you? Depends on how much battery life. And this case, I would definitely say that no, it is NOT worth it.

Sorry, I have to go now because my mom needs to use the computer.

Magikarp said...

I was actually wondering the same thing. Your post was very insightful. ~Thank you!

mangst said...


Thanks, I'm glad you liked it. Computer monitor technology has gotten much better over the years with regard to electricity consumption. CRT monitors (the big bulky ones) use much more electricity than LCD monitors (flat panels).