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I'm using a notebook as my only computer and I'm wondering how I might save the most energy (in terms of my electricity bill).

Will it use less energy overall to only plug it in if the battery is low and unplug it as soon as the battery is loaded? Or is it better to leave it plugged in until I'm done using it?

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Will it use less energy overall to only plug it in if the battery is low and unplug it as soon as the battery is loaded? I don't know if that will save energy or not, but that will quickly kill your battery life. –  Lie Ryan Jul 21 '11 at 19:50
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4 Answers

It's insignificant. Find better ways to save energy.

Your typical laptop consumes less electricity than one average light bulb.

This is a brilliant case of being penny-wise, pound foolish.

Laptop = 15-60 watts. Light bulb = 60-150 watts Fridge = 200-700 watts Dishwasher = 3500 watts Clothes dryer = 4500 watts ...

Are you planning to turn off your fridge every time it's cold enough and turn it back on just in time to avoid spoiling your food?

Energy saving should be an analytical activity. First, identify WHERE most of your energy footprint comes from, THEN figure out ways to reduce it. Don't fall for feel good, knee jerk measures.


Edit: Following your comment about electricity prices in Germany, here are some REAL numbers for you to think with:

Assuming 4 hours per day, 365 days per year, with a laptop consuming 50 Watts, you are using 76 kWh per year.

According to https://secure.wikimedia.org/wikipedia/en/wiki/Electricity_pricing:

1kWh = $0.306 (Germany - 2009).

76 x 0.306 = $22 per year.

If, through great efforts on your part, you saved 30% of your laptop energy consumption, you would reduce your yearly electricity bill by $7.

Do you really feel it is worth it?

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It's just one tiny piece of my general energy saving plan which checks anything that's consuming energy in my household. And that also includes the laptop. –  Benedikt Jul 22 '11 at 11:12
    
@user9673, In all fairness, you can't easily buy what you call "one average light bulb" in the USA any more, only fluorescents or LEDs that use much less energy. Benedikt, our generically-named friend's point is a good one: the time you spend trying to reduce laptop energy usage could be more effectively spent on higher-impact tasks like, say, shopping for a fuel-efficient auto (if you drive). For one hour of your life invested, you have to consider the return. –  CarlF Jul 22 '11 at 12:24
    
@Benedikt: Well, have fun with that but as far as your electricity bill is concerned, you wouldn't notice the difference if you were to stop using your laptop completely. It looks like your concern is money. If that's the case, you should consider that spending hours of your time to save 15cts of electricity per year is possibly not the best way to improve your financial situation. –  Sylverdrag Jul 22 '11 at 12:50
    
Sounds a little bit like you don't know the electricity costs in Germany ;-) –  Benedikt Jul 23 '11 at 6:16
    
@Benedikt: of course, my comment was tongue in cheek, but the point remains. I have updated my answer to give you some realistic figures to think with. Tell me if you think it's really worth your time. –  Sylverdrag Jul 23 '11 at 14:00
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To answer your actual question: discharging and recharging the battery, all other things being equal, will use more power than just being plugged in. Some energy is lost in the actual battery-discharge and battery-charge processes themselves, in addition to whatever you're actually using to do stuff on the laptop.

Other answers give good advice about how to actually lower your energy use.

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+1 for actually answering the question of discharging/recharging the battery -- the answer is that it uses more power than leaving the charger in when in use –  Soren Aug 11 '11 at 3:04
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  1. Turn the screen brightness down.
  2. Set the computer to automatically sleep/hibernate when idle (I have mine set to sleep in 5 minutes of idle time).
  3. Enable power-saving features of the CPU, e.g. Intel SpeedStep.
  4. Use smaller computer, a 11-inch Atom notebook uses less power than a 17-inch monster.
  5. If you need powerful graphics (e.g. for games), buy a notebook with dual VGA card, where one of the card is small power-saving card and the other is performance card. If you don't need powerful graphic, then don't buy a monster VGA card.
  6. SSD is more power friendly than a Harddisk, but they are also a bit more expensive.
  7. Spin down the harddisk, when not used.
  8. Avoid using peripherals, e.g. mouse, external keyboard, printers, etc all consume power. However, if using a mouse or external keyboard means you can finish a task in 1 minute instead of 10 then don't shy away from using them.
  9. Wifi, Bluetooth, and other wireless consumes power, disable them when you're not using them.
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I am downvoting this because, while everything you say is perfectly correct, the end result is a major hassle and it saves almost NO energy at all. The saving is insignificant. Doing what you recommend is like hunting down text files below 50kb on your computer and deleting them to "save space". In absolute terms, yes, you reduce the energy consumption. In practical terms, you are wasting your time and making life harder for yourself with no upside at all. If you were to turn off completely your laptop, I bet you wouldn't be able to spot the difference on your electricity bill. –  Sylverdrag Jul 22 '11 at 6:13
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@Sylverdrag -- seems unreasonable to down vote the answer just because you disagree with the question -- there is nothing wrong with the question either when viewed objectively, however your argument of "is it worth it" is probably very fair (I don't think it is either) but that should not affect the correctness of the Q&A -- however this answer does not answer the specific OP question of discharging/recharging the battery. –  Soren Aug 11 '11 at 2:59
    
@Soren: I downvoted it because it is a terrible advice to the OP. Seriously, would you recommend to turn the brightness down to save about $1.50/year? Will this benefit the asker? Potentially hurting his eyes and causing headaches for $1.50? Terrible advices should be downvoted, even if they are technically correct. Another example: SSD. Seriously? The guy is trying to save money and you are going to suggest SSD? The difference between a regular hard disk and a SSD is more than the consumption of the device over 10 years! No external keyboard/mouse? $0.10 per year. –  Sylverdrag Aug 11 '11 at 5:07
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@Sylverdrag: some of the advice is battery saving tips, which will also prolong the life of the battery, and since a lot of people dump their laptop when their battery no longer holds charge, prolonging the battery life will prolong the life of the laptop. And I do have a headache if I turned my brightness all the way up in the night. Also, half of the tips are about ensuring that you have the proper energy saving settings, there is no hassle to you at all apart from going to the Control Panel. –  Lie Ryan Aug 11 '11 at 9:51
    
@Lie Ryan: By default, reasonably savvy people adjust brightness to a comfortable level. You might like it darker than I do, but that's not the point. Going below that comfortable threshold causes trouble, whatever the threshold is for the individual. Lowering CPU speed = lowering both consumption and performance. If you didn't need the performance, it's no problem. If you do however... Using a smaller laptop also reduces usability and performance. Dual VGA card, cool. There goes another $3 per year. SSD of a decent size. Oups, all the cents you saved for the next 50 years are gone. –  Sylverdrag Aug 11 '11 at 10:09
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Type "cmd" on your "search for programs and files" and run the following command: powercfg -energy

After 60 seconds the program will create a html file that describes the possibilities to save battery consumptions and consequently energy.

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Assuming the OS is Windows –  Linker3000 Jul 21 '11 at 19:45
    
But does the notebook consume more energy if it's loading or if it stays plugged in when the battery is fully loaded? –  Benedikt Jul 21 '11 at 19:50
    
Exactly, the question didn't specify. –  Diogo Jul 21 '11 at 19:51
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Every single device on your notebook consumes energy, specially LCD and Wifi, the better you can do to reduce power consumption is turn off them(if you are not using wifi, turn off it, and if you are away from keyboard, set your Windows to turn off your LCD in few minutes, like 2 or 3). Your Processor and hard disk also consumes a lot of energy, however, most of current processors have the speedstep feature, that underclocks your processor on idle states; your hard-drive also can be turned off by your Windows when idle. Most that is possible will be given by the html genarated by the command. –  Diogo Jul 21 '11 at 19:57
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Upvoted because it's a great tip, but saving energy on laptop while plugged in is a fool's errant. –  Sylverdrag Jul 22 '11 at 6:29
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