Laptop Battery Life: How Wireless Affects Power Consumption

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Whenever we review notebooks one of the questions that always needs to be answered is, what's the battery life like on this laptop? We all know manufacturers overstate the quoted battery life for a system, probably because they test for battery life under ideal conditions for getting a high number. For example, wireless off, processor underclocked, system idle, LCD brightness set to low, no DVD and so on. So when your notebook with a quoted 5 hour battery life actually gets three hours, you're left wondering what happened to those other two hours the manufacturer got?

The reality is, a great number of things can affect the battery life of your notebook: what tasks you're performing, wireless throughput, processor speed, screen resolution, amount of file fragmentation on the disk, LCD brightness setting -- the list goes on and on. Over the next few weeks we're going to take a look at various laptop components, how they affect battery life, and what you can do to maximize or reduce the amount of power pull that component has on the system as a whole. We'll keep it at a high level so the average mainstream user can understand and use this information, and not have your eyes glaze over when reading. A majority of the data presented will be based on research and numbers compiled by Intel in their Power Optimization: Furthering the Mobile Vision series of articles, and presentations we've attended given by Intel, but we'll extrapolate that data to give real world suggestions.

The Effect of Wireless on Notebook Power Drain

In case you wondered what a wireless card looks like -- above is the Intel 4965agn wireless card (view large image)

It's now the year 2007 and 99.9% of notebooks out there have built-in wireless allowing you to surf the web and transfer files over thin air in the form of radio waves (802.11 a/b/g/n). Having built-in wireless means you've got a little chip inside your notebook that's responsible for sending and receiving those wireless packets of data. If you're viewing this web page and on a wireless internet connection, the wireless chip in your PC was required to work and exert a certain amount of power to download this page. If you're plugged in, that power is drawn from your wall socket, if you're on battery, then a very small piece of your battery life was donated to downloading this page (we hope it's worth it for you).

So, exactly how much does having a wireless radio on and surfing the web affect your battery life? And how much power can you save simply by turning it off? We'll investigate in this article.

Below is figure 1 that we'll refer to a lot. It's a bar graph showing how much power is exerted by each component in a notebook. There are five bars, each showing how much power is used based on what the notebook is doing. It's important to note this benchmarking system was done on a reference system that did not include an LCD monitor. We'll look at LCD power drain seperately in another article (it is generally the largest drain on battery life).

Figure 1: Amount of battery drain of a notebook in various usage states. LCD power drain is not included, the reference system does not include a monitor.

 

Below is another graph that shows average power drain for four different states of wireless, which are documented below this graph:

Figure 2: Power drain of idle notebook running Windows XP with various WLAN states. Blue indicates power used by all othercomponents, except the LCD power drain which is not included, and purple represents WLAN power drain.

1. Power drain of idle notebook (screen power drain not included) with wireless radio Off = 0W wireless radio power consumption
2. Power drain of idle notebook (screen power drain not included) with wireless radio On, but not connected to an Access Point (no data being sent or received) = 0.30W wireless radio power consumption
3. Power drain of idle notebook (screen power drain not included) with wireless radio On, connected to an Access Point (data can be sent and received) = 0.45W wireless radio power consumption
4. Power drain of idle notebook (display power drain not included) with wireless radio On and Searching for an Access Point = 1.6W wireless radio power consumption

Notice that by far the most power drain from the wireless card comes when it is searching and trying to acquire an access point. Let's calculate a worst case scenario for how much the wireless card could affect battery life of a notebook. Meaning, let's assume the card is perpetually searching for a network and can never find one, so the increased power consumption is +1.60W, therefore:

• Assume average power consumption for notebook with no wireless is 20W
• Assume average power consumption for notebook with wireless, continually trying to acquire network is 21.6W (+1.60W)

The formula for battery life (assuming a brand new battery with no wear on it) is:

• Battery Life = Battery Capacity / Laptop Power Consumption

So if we assume a battery capacity of 53 WHr (fairly standard battery capacity) the formula for battery life with no wireless is:

• Battery Life No Wireless = 53WHr / 20W = 2 hours 39 minutes

And the formula for wireless on and searching for a wireless network is:

• Battery Life Wireless on and Searching for Access Point = 53WHr / 21.6W = 2 hours 27 minutes

So the difference in battery life for this worst case scenario is 12 minutes, which amounts to about 7.5% difference. Now let's assume the 0.45W power drain for the case where wireless has a solid connection and we're just transferring a little bit of data:

• Battery Life Wireless Connected = 53WHr / 20.45W = 2 hours 35 minutes

Not bad, only 4 minutes, or 2.5% overall difference!

But, can we always assume 0.45W power drain from wireless? What if we're uploading and downloading a ton of data, does that affect power drain somewhat? Well, yes...

Wireless Power Consumption Based on Data Transfer Variance

We're going to look at a comparison of transferring compressed versus uncompressed files to demonstrate the effect the amount of data transferred has on power consumption.

You need to be aware that the more data you transfer, the more the wireless card has to work, meaning the more power it has to consume. So if you're streaming several gigabyte video files from a media server in your house to your unplugged notebook, your wireless card will be sweating hard and doing a lot of work, thereby draining battery power faster. If your notebook is on wireless, solidly connected, and all you're doing is checking email every 5 minutes, the wireless card will barely sip the power and have little effect on battery life.

Let's look at how amount of data transfer affects energy used with some charts and graph comparisons.bbAssume we have 5 different files, each file can be compressed to varying degrees using GZIP:

Data Set	Original Size (KB)	Gzip Compress Rate (version 1.2.4)	Description
Tulips.tif	1179 (1.15MB)	1.2x	Med size file, very low compress ratio
Book1	751 (0.73MB)	2.45x	Med size file, low compress ratio
World95.txt	2935 (2.87MB)	5.06x	Large size file, high compress ratio
Pic	502 (0.49MB)	8.96x	Small size file, high compress ratio
Frymire.tif	3708 (3.62MB)	14.04x	Large size file, very high compress ratio

 

Tulips.tif is almost incompressible, while Frymire.tif can be compressed to almost 14x smaller than its original size. Here's a chart showing how long it takes to download the uncompressed file versus downloading a compressed version of each file plus the amount of time it then takes to then uncompress that file once it is downloaded:

Notice that with Frymire.tif we save about 2.5 minutes by downloading the much smaller zipped up version of the file and then unzipping it. Meanwhile, the Tulips.tif file is about the same size compressed as uncompressed, and then we have to spend time and processor power unzipping it, so it ends up taking more time and power overall!

The amount of time it takes to download a file and then uncompress a file has a very direct effect on power consumption. Notice in the graph below how much less energy it takes to download the compressed version of Frymire.tif:

Several times more energy is consumed to download the uncompressed original 3.62MB size Frymire.tif, it took almost 3 minutes to download Frymire.tif, so the wireless card is working all that time.

When you download a web page, most websites (such as this one) actually send a page in gzip format and your browser will unzip it on the fly. So this type of thing is constantly going on behind the scenes as you browse the web. The more web pages you download and browse, the more power it takes.

The moral of the story? The more you're downloading with your wireless card, the more power it consumes, so the effect your wireless card has on battery drain depends a lot on how much you're using it.

Conclusion

For those that saw a couple of formulas and graphs and skipped ahead to this part, I don't blame you, here are the take aways based on the data:

• Simply turning on the wireless card doesn't drain much power, it might shorten battery life by 2%, so if you want to remain connected to download email it's no big deal.
  
  
• If you turn on wireless and are constantly trying to acquire a connection, such as if you're constantly dropping a connection and then trying to reconnect, your battery life could be 6% - 7% shorter than usual due to the wireless card having to work extra hard.
  
  
• If you have wireless card on and are transferring lots of data, such as downloading large files or constantly browsing the web, battery life could be about 6% shorter than simply no wireless on. Consider downloading large files later when you're plugged in.
  
  
• If you don't need to use the web, turn wireless off, it'll save you at least a few short (but maybe precious) minutes of battery life!

Overall I believe that power consumption due to wireless is a bit exaggerated as a culprit for notebook battery drain. You can save a lot more power by dimming screen brightness, underclocking the processor, spinning a DVD less, or a variety of other power saving tactics we'll look at in future articles. In most cases you're going to have a lot more fun or be more productive when connected to the web, so don't sweat it, turn the wireless card on and be more productive with that time you have on battery.

 

Well it seems to not affect battery life too much at all.

 

What would be better?

Turning off wireless and turning it on to check e-mail say every half hour, or keeping it on and having it check automatically? Like using Gmail notifier or something.

If you turn it off and then turn it back on, it has to find the access point again, but it saves power while it is off.

If it is on all the time, it is constantly taking power.

Thoughts?

 

That's a very interesting article! Turning off Bluetooth and WLAN had a noticable effect on my Dell X1. It usually lasts 4 hours, but turned off it was 4:15-4:20.
But I guess that included Bluetooth, so the article is very precise.

Now we need an article about screen brightness and battery life!

 

That'll be coming in the next few weeks!

You'd probably break even on this, if you turn it on and off every ten minutes it's best just to leave it on. If you're doing it every 30 minutes you might sae a minute or two of battery life.

 

Quite informative. Looking forward to all of the following reports.

 

Nice article. Thanks. Looking forward to the rest of the series...

 

Nice article, very informative.

Grammer police: effect (a noun, usually), not affect (verb)!; But affect in the title.

 

Spelling police: Grammar, not grammer. Use Firefox 2.0.
--Sorry, I had to do it, no hard feelings I hope.

Very interesting article Andrew. I never really figured that wireless would be a big battery killer. To be quite honest, I'd say a big culprit for most people on these forums would be a dedicated graphics card, which can use anywhere between 5-10 watts or 100 watts for the Quadro FX 2500M/3500M at full clock speeds. Mine consumes 45W at full speed, as does the 7900GTX.

 

Thank you Andrew, honestly I spent the whole of yesterday trying to improve battery life on my V1JP. I have found that disabling the webcam in the device manager helped. I found myself consuming 19 watts in class with Wifi on, but no connection because there is no signal there.

Oddly enough it is really hard to reproduce the same consumption levels, I went home to show my parents this and they were surprised I was able to keep my consumption so low, doing light tasks with wifi on, but not using the internet.

I plan on writing a follow up review on my V1JP, hopefully completed by Sunday, with new battery life numbers and tweaks and everything, which can hopefully help those who are considering the V1JP and those who own it, to get the most out of their laptop.

I have also found that if my laptop brightness is at 2/16 and then i change to 3/16, i consume 0.5 watts more. But changing the brightness from 2/16 to 3/16 is noticeably brighter. hehe

If anyone can think of other devices that should be disabled to increase battery life that would be great!

Thanks,

MysticGolem

 

Thank you notebook_ftw—no, no hard feelings.
I thought I might be an ass when I posted it, now I know it!

Yah, Fx 2.0 rules!

 

I usually disable both the integrated camera as well as my 1394 controller, onboard nic, onboard modem. Can always re-enable them later if needed. Not sure of the battery life impact though cause it's always been that way for me.

 

Great research here, Andrew.

I actually didn't suspect the drain on battery was so minimal...it's good to see the data.

On a random note, I'd like to see someone compare power consumption and range statistics on Atheros v. Intel cards.

 

Well, now i know why my classmate always had his wireless turned off on his Acer notebook. It saves a couple minute's worth of power. But for me, after reading that article this morning, i decided to devise a plan for my laptop to convserve power in school.

Thanks for the article!

 

Very interesting..

I also did a similar battery test at school. Every week I have a 3 hour lecture which requires a laptop to take notes.

With brightness level 3, no sound, no DVD, no LAN, no WiFi, no Firewire, no Bluetooth

I get almost 3 hours and 45 minutes of usage.

With everything remaining the same but with WiFi on and web surfing, I get just a little under 3 hours. My laptop usually shutdown 5 minutes before class ends.


There is something I don't understand about my laptop. I have a switch near my power button which allows me to turn on or off Wireless. Is the switch exactly the same as going to Wireless Network Connection and click disable.

 

Thanks for an excellent article and I look forward to the subsequent parts of the series. The subject is one which I have looked into in some detail for my own notebooks while searching to extend the battery life but the presentation here is more complete and structured. For people with XP or earlier you can used MobileMeter to monitor your own battery drain. I don't know if it works under Vista.

Something to emphasise is that "average" power consumption is difficult to define since power usage is not a symetrical distribution about the average. There could / will be bursts of activity which temporarily increase the power drain by 50 - 100% of average, but there won't be corresponding reductions.

There are a couple of reasons why batteries run out before the predicted time being reached: (1) The computer is usually set to shut down before the battery is fully drained (the default is usually 3%); and (2) the battery is not delivering its full capacity due to wear. There is usually a calibration utility somewhere (maybe in the BIOS). Unless the power management system knows that the battery has lost capacity then it will count down from full. MobileMeter provides the information on battery capacity and wear.

John

 

This is at odds with my own experience: having wireless on with my Dell D820 roughly halves battery life.

I notice that the article does not mention what wireless protocol is employed (a, b, or g), nor what encryption is used. Certainly WPA-PSK will work the CPU a lot harder than WEP, which in turn chews more power than unencrypted.

Andrew, can you please specify what protocol and what encryption you used ?

 

That's right! Some network cards (ethernet and also wireless) have more hardware support for data transmission tasks (like CRC checks, I am not sure about encryption) and other cards are using drivers to do the same things...