Measured power usage
Measured with Kill-A-Watt, 119V input, tested with 720p output to LCD (not plugged in to wattmeter)
Note: These figures have also been confirmed with a 240V unit.
- Main menu: 14-15W
- Playing a 640x480 XviD file: 17-18W
- dnetc -benchmark2: 20W
- "Sleeping" (white light off, no video output after holding down Play button): 14-15W
- After a "shutdown -h now" command: 15W
- Is there any way to further reduce power usage, e.g., disabling the graphics processor?
- Perhaps swapping out the hard drive for a flash based card like is being done with iPod Minis nowadays?
- What about a "Shutdown" frappliance that physically turned it off. (Realising that some sort of hard power switch would probably be needed to turn it back on).
Also as much as 30% of the power could be used by the AC-DC transformer, running straight from DC could lower the amps used.
- Doing a pmset -g yields:
-bash-2.05b$ pmset -g System-wide power settings: SleepDisabled 1 Active Profiles: AC Power -1* Currently in use: disksleep 15 hibernatemode 0 displaysleep 1 powerbutton 0 sleep 30 autorestart 1 hibernatefile /var/vm/sleepimage
So it looks like sleep has been "disabled" in the sense of traditional mac sleep. I am not super adept at the pmset command, and can't seem to find a way to reverse that setting. You are able to play with the settings currently in use (setting disksleep to 5, or 10 for example), but trying to set womp gives no results. A pmset -g cap gives you:
-bash-2.05b$ pmset -g cap Capabilities for AC Power: displaysleep disksleep sleep autorestart powerbutton
Additional, you can directly edit the file: /Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist. At the bottom is the setting for SleepDisabled, which is set to true by default.
Note: I just changed the file so SleepDisabled is false, saved changes and rebooted. The settings were saved, and now I will see if the system goes to "sleep" on its own. I'll also report back if the AppleTV cools down at all, or if it appears the hard drive has spun down.
- Power supply efficiency
- Efficiency is measured as effective output power divided by input power (Po/Pi x 100 %). Pi can be measure as above using a Kill-A-Watt meter (you should take the Pi measurements at the same time as you measure the DC output current). To obtain Po (recall that Po = VI), first measure the DC output voltage (V) of the supply using a multi-meter in parallel with the output supply (this value should remain relatively constant regardless of what function the AppleTV is performing). Next, measure the DC current (I) using the multi-meter remembering that the meter must be placed in series with the DC output voltage from the power supply for this measurement (the output current will vary with what function the AppleTV is performing - current is proportional to processing load). Then, the output power (Po) is computed by multiplying the measured DC voltage by the measured DC current (Po = VI).
- The digital wattmeter I have can measure P and U*I. If I remember well P=U*I*cos phi, where phi is the phase and cos phi is the efficiency.
For AppleTV I have measured in idle P=15 W and U*I=31 VA, the efficiency seems to be comarable with my laptop power supply, where I have P=30 W and U*I=60 VA.