17881 Views 8 Replies Latest reply: Jan 26, 2008 11:43 AM by rwinkel
Suggestion: spend a little money and get a Kill-A-Watt. Most IT guys know what it is, but you can Google it.
OK, I just plugged my 2.0Ghz Core2Duo Macbook into my Kill-A-Watt. I was running off battery earlier today, so it's charging now, and indicating that it's consuming about 37 watts. When I unplugged the MagSafe from the Macbook, the power adapter consumed no power.
Later, after it's fully charged, I'll report how much power is consumed when just browsing - but that number can jump around a bit.
I pulled out my 1940s era General Electric Wattmeter. This is a two-coil instrument that measures Vrms * Irms * cos( phase angle) = actual Watts. This type of meter does not measure reactive power that does not deliver energy to the load but circulates in the system. Reactive power is a concern because the out-of-phase current of reactive power does cause energy loss (I squared R loss) by heating the wires carrying the current.
Power brick energized, but not attached to computer - unmeasurable.
Power brick energized, attached to computer, green LED on - about 0.7 Watt.
Power brick energized, attached to computer, OSX booted, but idle - 8 to 10 Watts.
Power brick energized, attached to computer, screen saver running - 8 - 12.5 Watts.
Power brick energized, attached to computer, 5th Element DVD playing - 12.5 -15 Watts.
Power brick energized, attached to computer, Grapher, contours.gcx - 18.5 - 20.5 Watts.
Once in a while, I could get the MacBook to peak at about 25 Watts (averaged by the mechanical dynamics of my meter) of power consumption. The initial plug-in of the power pack to the socket also consumes several amps for a split second - too hard to measure accurately with my crude equipment. There's also a little audible crack! as the prong touches the outlet, so it is obviously a pretty large momentary in-rush of current.
When the power brick is unplugged, the Irms current draw is about 22 mA according to my Fluke 8060A. Presumably that's almost all reactive at close to -90 degrees phase angle relative to the voltage. Switch mode power packs like the MacBook's tend to be capacitive loads. That's about 2.64 VARs of reactive power.
When the MacBook is idling at about 9.5 Watts of real power consumption, the current draw is about 245 mA. The real power corresponds to about 70 mA of in phase current. This gives the surprising result of about 234 mA of lagging reactive current.
In terms of what real-world thing can you do? For 10 to 100 MacBooks, I don't belive it is worthwhile to do anything. Probably remediating the power supply to reduce the reactive current load would be good thing. At industrial scales, factories worry about current phase angle a lot. Power providers bill for reactive current they have to generate, otherwise they would lose revenue by heating their transmission lines, but the customer gets no energy delivered to the manufacturing process by the out-of-phase reactive current.
Probably, the energy consumed and eventual environmental waste generated by remediating 10-100 power supplies, not to mention the engineering hours required, would not offset the power that would be saved over the useful life of the MacBook.
On the other hand, grumbling at Apple to contract with the manufacturer for a greener design for the power pack for the many thousands of users would be a good thing on which to expend a little energy.
PS: just to be sure my Wattmeter is giving reasonable results, I plugged in a "60 Watt" bulb that's been used for about 200 hours. According to the meter, it draws 58 watts.
Message was edited by: impulse_telecom
On a related note, I have a new macbook model 2,1 which comes with a 5300 mAh 12V battery. I have 4GB of ram installed. In sleep mode with no power adapter, it depletes the battery at around 1% of capacity per hour. This works out to 0.636 watts of power consumption in sleep mode. This is higher than I expected. Is this normal?
In looking at the "about this mac" -> "more info" screen, under the power category, it says the sleep mode timer is 5 minutes, the display sleep timer is 3 minutes (ok so far) but the disk sleep timer is 10 minutes. Is this a problem with the info display or with the bios?