Reduced desktop power consumption

Monitor: 75 w, speakers: 11, DAC: 12 w, cpu: 290-400 w

I recently used the tools of DMAIC (Define Measure Analyze Improve Control) to reduce the energy diet of my home computer.Through careful planning and not a little bit of cleverness, I was able to cut power consumption in this area by 64%, based on a draw of 1985 kWh/year. I step through the entire DMAIC process, including my future control plans.

This is part one in what I hope to be a continuing series of examples regarding using DMAIC to solve energy problems. I intend to conquer my entertainment section next.

Define:

My computer’s power consumption is too high. I want to serve files and keep a screen session with my IRC client up, but I don’t want to waste energy. I need to find a way to serve files and waste less energy.

Measure:

I’ll spare the minutiae and just focus on the essentials. Since I was serving files, the system was consuming its base power at all times. There was no sleep to speak of. If you did sleep the computer, the whole system would still draw 32 watts with the peripherals on standby. This is the reason I made the changes I did later.

My power consumption, based on never being able to turn the system off.

• Off: 28 watts
• Sleep: 32 watts, but the best we could do was 210 (system on, monitors and speakers off)
• Powered: 290-420 watts

Analyze:

• Active consumption is around 347 watts, deviation 40 watts
• Off consumption is high at 28 watts
• Sleep consumption is high at 32 watts
• I Can’t sleep while the system is serving files
• I needed to stop standby losses
• I need serve files without the main system being powered on.

Improve:

There are a few quick habits I could add to my routine to save a bit of power here and there. I could have the system put the display to sleep sooner, saving 70 watts during system uptime. I could turn the monitor off before the system disables the display for another 5 watts savings. I could shut the speakers off when they aren’t in use, saving another 5 watts.

At this point I was still serving files, so I couldn’t put the system to sleep. The next step was a bit counter-intuitive. I added a server to the systems on my desk. The system is a Mac Mini, and it draws 19-22 watts while on. Even though it increases power consumption up while the main system is on, it is worth it because I can sleep and shut off the main system when not in use.

I have one more trick up my sleeve to ‘even things out’: a Smart Strip. A Smart Strip is a power strip with a master socket and a set of slave sockets. The slave sockets are only powered so long as the master socket draws power. So, when my system is off or asleep, the maximum draw for the main system is 1-7 watts (1 when off, 7 when asleep) Since the total draw of all those components when idle before this step was 26 watts, the mini’s power is basically paid for by the absence of their draw.

My power consumption after offloading some tasks to a low-wattage server.

Assumptions:

Cost/kWh: $0.10
Hours used: 5/day

Before: Serving files on main system. effectively, Standby was system consumption minus monitor and speakers.

After: Offloaded serving files, cut off devices when main system is low power state. Standby is literally “lights out” for the main system.

Moving the files to the mac mini and deepening hibernation saves me $120/year.

Control:

My intent is to build a Tweet a Watt kit and have it save results on the mini to track actual use and further refine the process. If this project is successful, I will be doing the same thing with my Entertainment center. I currently shut that off at the power strip.

In order to control further my energy use, I may purchase a laptop for general daily use instead of using the Mac Pro all the time. For one thing, laptops draw 110 watts maximum when plugged in, and you can unplug them and transport them to different locations. This isn’t about saving money, it’s about saving energy. I don’t really care about the money part as much as the carbon footprint and our natural resources.

My main system would stay off except for gaming, photoshop and other related tasks. This would reduce the uptime of the main system, at the cost of some idle charging. I think this change would reduce the cost per year by half.

An additional note: Apple’s official numbers for power consumption on the mac pro (all revisions) and the mac mini