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I am thinking about installing the free Avast anti-virus software on my MacBook Air (2012). Once Avast has completed its first scan of all of the files, does its continued operation noticeably affect battery life?

Is the answer to this question known for alternative anti-virus programs?

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I tried installing Avast and adding this to crontab:

* * * * * ps -o \%cpu= -p $(pgrep -i com.avast) | awk '{s+=$0}END{print s}' >> ~/Notes/avast.txt

The average CPU use (for one core) after about a day was about 1.5%, but I'll keep it running for a few more days or weeks.

iStat Menus displays CPU power consumption in Sensors > CPU Package Total. Approximate CPU use and power consumption on my iMac:

  • 5%: 5W
  • 100%: 13W
  • 400%: 25W

So 1 percentage point increase in CPU use (for one core) from being near-idle might correspond to 1-2% increase in CPU power consumption. The ratio might be higher for laptops though. And the percentage change in total power consumption depends on the power consumption of displays and other components.

Idle power consumptions with display off or at maximum brightness:

  • 11-inch MacBook Air: 4.7W, 9.0W
  • 15-inch MacBook Pro with Retina Display: 5.3W, 17.6W
  • 21.5-inch iMac: 14.1W, 41.8W
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Wow, iStat Menus is awesome! Just bought it. Thanks for the pointer. –  Mark Adler May 28 '13 at 5:19
    
Perfect. This data and iStat on my machine give me the information I need to answer my question. –  Mark Adler May 28 '13 at 5:47

In general; yes. Most anti-virus programs do on-demand scanning, meaning they check all files as they are being requested by various programs. This real time scanning requires drive access, processing and some memory, all of which contribute to a slower machine with less battery life.

However, I'd heavily advice against the use of anti-virus programs on Macs. Anti-virus programs for Mac usually don't have any Mac virus signatures that they check for. The main point of these programs is to check for Windows viruses, and prevent accidental infection of coworkers'/family/friends' Windows PCs. Mac anti-virus programs tend to be scams, which although may offer legitimate detection of Windows virus, are simply meant to pray on users and push them into purchasing a paid product.

Apple does a GREAT job at patching up various vulnerabilities, so you should rest assured that your machine is very secure, as you're likely not a high profile enough personality to be worth the effort to hack.

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I know it must affect it at some level. The question is, from experience, does it noticeably affect battery life? E.g. at about the 10% or more level. –  Mark Adler May 27 '13 at 4:41
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That would be an impossible measurement to make. If I told you it uses 10% more battery, is that with the display brightness all the way up or at minimum? WiFi on or not? There are too many factors that effect battery life for a concrete answer to be possible. As I stated in my answer, I strongly recommend against the use of such software unless you frequently exchange files with other PCs. It's a waste of system resources no matter how you slice it. –  XAleXOwnZX May 27 '13 at 5:58
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Impossible? Really? It would be a rather simple experiment to do with the anti-virus program turned on and turned off. Apple has defined a standard for their battery life specification: "The wireless web test measures battery life by wirelessly browsing 25 popular websites with display brightness set to 50%." –  Mark Adler May 27 '13 at 15:26
    
Thank you for the recommendation, but my organization does in fact want to protect itself from Macs as a Windows virus vector. –  Mark Adler May 27 '13 at 15:28
    
""The wireless web test measures battery life by wirelessly browsing 25 popular websites with display brightness set to 50%." That's the type of thing you state in your question… I guess I didn't mean "impossible", it just requires the data that you just gave me. I suggest you append this to your question. –  XAleXOwnZX May 27 '13 at 22:29

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