Apple Laptop Power Consumption Tests

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kill-a-wattAt Christmas 2005, Santa brought me a Kill-A-Watt. Apparently he ordered it online at

Since I am an engineer, the result of possession of this gadget was entirely predictable. Someone once said "To a man with a hammer, all the world is a nail." I proceeded to start measuring things.

The Kill-A-Watt measures AC line voltage, current, true RMS power, VA (reactive power), line frequency, power factor and kilowatt-hours over some number of hours for 110 VAC loads up to 2 kW. It's least significant digit in the true RMS power mode is 1 watt. It's least significant figure is 10 mA in the current mode, but at 110 VAC, this is also 1 watt.

Since I had been obsessed with the battery life and power consumption of laptops (PowerBook Battery Life and Some Thoughts on Computer Upgrades), I figured that I'd get some data.

I had 3 computers to test and I elected to test them in several different modes while running without a battery and several other modes while charging a fully discharged battery. The testing without the battery was done so that there was no ambiguity about any residual charging power the battery might be taking even if the computer declared that the battery was charged. If there was no battery installed, there was clearly no power being directed to it.

AC Power Consumption Of Some Apple Laptops Without A Battery Installed

The normal power consumption tests were done in several states selected to produce the most, least or typical power consumption. With the standard Apple 65 watt power adaptor disconnected from a computer, the Kill-A-Watt reads zero watts.

The Kill-A-Watt displays a new reading every second. In the cases where the readings alternated between two values, I estimated the power right between the displayed values, for example, 12.5 watts instead of 12 or 13 watts.

Reference State. The reference state was chosen to represent a "normal" operating state. Other measurements are also in this state except for the feature being tested. The conditions are:

Reduced. Same as reference but processor performance set to "reduced."

Zero Brightness. Same as reference but with the backlight completely off.

Minimum Brightness. Same as reference but with the backlight set at 1 tic mark.

Airport Off. Same as reference but with the airport turned off. Note that the PowerBook and G4 iBook have Airport Extreme. The G3 iBook has a regular Airport card as it does not support Airport Extreme.

Sleeping. Same as the reference state but sleeping.

Best Power Conservation. Settings intended to produce best battery life:

Pocket Tanks. Same as the reference state but with Pocket Tanks running. This game has a history of running a computer as to the highest power consumption that it will go without additional disk accesses.

Peak Power During Boot. The highest reading shown during the boot process up until the login screen is displayed.

Peak Power With Login Screen Showing. Ready to login into the reference state.

Peak Power During Login. Logging into the reference state. This tends to produce the highest peak power consumption as the CPU can be working very hard AND the disk is being hit hard as well.

Shutdown. You'd think that this would be zero but it isn't.

AC Power Consumption with Battery Removed
All Power Levels in Watts
State 15" 1.25 GHz G4 PowerBook 12" 1.2 GHz G4 iBook 12" 500 MHz G3 iBook Notes
Reference 24.5 16.5 15.5 Activity Monitor running to allow CPU utilization to be judged. CPU activity less than 10% and power consumption is stable.
Reduced 22.5 15 15.5 Activity Monitor running to allow CPU utilization to be judged. CPU activity less than 10% and power consumption is stable.
Zero Brightness 18 12 11 Not a useable condition, but it provides a reference for the next state.
Minimum Brightness 20 12 11 Useable in a dark environment.
Airport Off 23.5 15.5 15 Older Airport card may draw a little less power than the Airport Extreme
Sleeping 2 2 2 There must be some keep alive power to keep the dynamic RAM refreshed.
Best Power Conservation 16 11.5 11.5 Good for traveling when the lighting allows.
Pocket Tanks 40 25 21 Heavy CPU but light disk usage. Other heavy activities are importing photos to iPhoto and rendering movies in iDVD (which I caught at 44 watts peak with both fans going.)
Peak Power During Boot 32 20 21 Heavy disk usage.
Peak Power with Login Screen Showing 24 15.5 15 User processes not started, typically won't charge in this state.
Peak Power During Login 41 25 24 Heavy CPU AND disk utilization.
Shutdown 2 1 2 Should be less than this. Keep alive power is not needed except to run the clock.

There are some interesting observations to make from this data. First, the older iBook doesn't appear to respond to the reduced processor setting at all and the new iBook which is 2.5x faster by clock speed alone doesn't draw significantly more power than the older one. Either a G4 is more power efficient than a G3 or Apple got smarter in designing power conservation features in the newer computer.

The PowerBook is a PowerHog. Its got a lot more RAM and a bigger display and disk and a more powerful graphics processor which all contribute to its higher power consumption.

On all three computers, the display backlight consumed 4.5 more watts from the minimum brightness setting to the full brightness setting. I expected the PowerBook to actually do worse.

On any of them, going conservative on power settings can increase your battery life by about 50%.

None of this has anything to do with the battery, when you've got a dead battery to charge too, the power can really go up, see the next section.

Power Consumption While Charging A Dead Battery

I also tested the power consumption when a fully discharged battery is installed. A slightly different series of tests were run.

When the initial tests were run with dead batteries I observed that the green/orange indicator on the charging plug really means that it is actually charging at a high rate or not. The power consumption would change abruptly when the light color changed. However, when the battery is near full charge, a green indicator didn't mean that the battery wasn't getting some charge. This is when I determined that I needed to pull the batteries out of the computers to make sure that none of the indicated power was actually going to the battery during the testing described above. This was especially noticeable with the G3 iBook. Also, when the battery indicator is indicating "Calculating...", the battery may or may not be charging at a low rate. Also, as the battery continues to charge, the total power consumption seems to be constant through the first part of the charge cycle.

Total Power While Charging A Dead Battery
State 15" 1.25 GHz G4 PowerBook 12" 1.2 GHz G4 iBook 12" 500 MHz G3 iBook Notes
Reference + Charge 65 43 26 The degree of charging allowed varies greatly by model.
Shutdown + Charge 40 27 30 PowerBook charges at a much higher rate than either iBook
Sleep + Charge 41 27 30 Same as shutdown
Boot + Charge (Green) 27 20 25 It would appear that when the light is green, the battery isn't getting a lot of charge or any at all.
Boot + Charge (Orange) 65 N/A N/A iBooks didn't want to charge during boot.
Login + Charge 65 48 32 When the disk is being hit, the total power can increase a little more.

What is pretty clear from this data is that Apple has changed the design of the battery charging circuits from model to model. The PowerBook runs right up to the rating on the power adaptor. As an additional note, while the PowerBook was charging at 65 watts from it's adaptor, I substituted an older 45 watt brick. The total power abruptly dropped to 46 watts. Plugging the 65 watt brick back in brought the charging power back to 65 watts. However, all of the tests above on the iBooks were also done with the 65 watt brick and they would not draw the full capability of the brick in any case.

Near the end of the charging cycle, with the battery capacity up to around 70%, the charging power starts to decrease and tends to taper off during the last part of the charge.

It has always irritated me that I never got nearly the battery life advertised by Apple out of any of my computers. Apple had to have a basis for their claims or they could get sued. After running the power conservation tests above, it occurred to me that the power consumption under those conditions was nearly low enough to get the claimed life with a good battery. Neither of the batteries in the PowerBook or the older iBook are in new condition, but I decided to run the test anyway. What I got surprised me. I actually validated Apple's claims.

Apple never claimed that their advertised battery life could be obtained in real world conditions. The claim merely indicates that the advertised battery life could be obtained under some condition. The best conservation case above requires that you be in a dark room and not do much of anything to keep the CPU utilization down, but the damn things will indeed run for more than 4 hours. With a new battery, this might stretch 10% longer.

I tested the older iBook first. It's battery is about 10% below new capacity and it ran for 5 hours and 45 minutes AND the predicted life was fairly accurate, within a few minutes of the final run time. The disk cycled on and off all during the run spending about 50% of the total time spun down. This was shown in the recorded XBattery current consumption plot.

The PowerBook's battery is about 10% below it's new capacity but it is an aftermarket battery which had about 10% more capacity that a stock Apple battery to begin with. Now it about equal to a new Apple battery. It ran for 4 hours and 17 minutes. The predicted run time was also accurate as the battery had just been calibrated the day before in preparation for the charging current tests. However, the XBattery current plot indicated that the disk never spun down at all.

At the beginning and end of the discharge for both systems, I recorded the current and voltage to get the power consumption. The actual power was nearly constant as the current was increasing to compensate for the decrease in battery voltage. In both cases, this indication of power was about 80% of the power that was measured through the power adaptor. However, even if I take these lower numbers and multiply them by the time of the discharges, I still get a higher watt-hour number that the batteries are specified to provide. The batteries are specified at about 45 watt-hours, but I got 54 watt-hours for the PowerBook battery and 56 watt hours for the iBook battery.

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© 2005-2007 George Schreyer
Created 27 Dec 2005
Last Updated February 18. 2007