One of my biggest hobby electronics projects thus far has been the development of a solar power system, used primarily for home automation (for details see Part I and Part II). In the past, I’ve used it to partially power my five-year-old ThinkPad T43p laptop, which also acts as the brains of the system. Unfortunately, given the limited number of photovoltaic (PV) panels I possess (not to mention the lack of a good mounting location for these panels), I could only run the laptop off-grid for about six hours on a sunny day.
Now that my PV system is (mostly) inactive, and I’ve got my Kill-A-Watt meter back, I’ve taken some time to quantify just how far I can reduce the power consumption of my T43p by disabling components. While I was at it, I ran the same tests on my more modern ThinkPad T61p. And if you’re like me, I think you’ll find the results rather interesting.
However, before I begin, I must tell you that this morning I’m mourning the loss of my ThinkPad T60. What, another laptop? Yes, admittedly, I have something of an obsession with ThinkPads. I bought the T60 as a replacement/upgrade for my T43p. I got a great deal on it from a guy on EBay, too – just $230 for a mint condition, perfectly functional PC. Even the battery still held about 80% of its design capacity. Unfortunately, after owning and operating my T60 for about six straight months, it shut down and now won’t start.
I’m not really sure what’s happened; I wasn’t around when it failed. I had it running as a server in a remote location for several weeks. Then one day I just couldn’t connect. It appears the motherboard has been damaged somehow, but certainly not from any physical impact. Perhaps an electrical storm? I’ve tried everything – pressing the power button 10+ times as some forums have suggested, removing components, etc. It just won’t turn on. When plugged into AC, the battery and plug lights come on, but nothing more.
Well, at least I can still use it for parts. I’ve already swapped one of its 2GB RAM sticks into my mom’s virtually identical T60. And the battery fits my T61p, so now I can carry that along as a spare. I may look into buying a new motherboard, but it’d probably cost me another $80-100. Oh well… so far this is the only blemish on my ThinkPad experience.
Testing Power Consumption
In order to perform these tests, I’ve used a P3 Kill-A-Watt power meter. This is a super handy gadget for anyone interested in determining how quickly their devices are using electrical energy. It’ll measure up to 15A at 125VAC. In addition to current, voltage, and real power, this device also measures frequency, VA, and power factor.
I should quickly mention that while I trust the Kill-A-Watt’s real power measurements, I’m not sure it accurately computes power factor and VA for devices with non-sinusoidal current draws. And most laptop power supplies draw a very distorted current waveform due to the nature of their input rectifiers and capacitance. Here’s one example I recorded a few months back using the supply for my T61p and a nice hall-effect current probe:
The Kill-A-Watt claims this signal represents a power factor of 0.59. Although I haven’t crunched the numbers, my guess is that this is a little on the low side. Of course, I’m no expert in power-factor correction, particularly with such odd waveforms…
Well let’s get on to the good stuff. During this testing, I always allowed my laptops to reach a steady-state condition before recording anything. Thus, I took no data during boot-up, program launches, etc. I also ran all tests using a fully-charged battery. The following numbers would of course be somewhat higher if the battery were charging. However, just how much higher they’d be may vary depending on the battery’s state of charge. So in an effort to eliminate some variability, the battery in each laptop was first fully charged. The data I collected was then condensed into one simple graph (click to enlarge):
Now you can look at this however you like, but I prefer to read it from left to right. So beginning on the left, in blue, you’ll see that the T43p draws a minimum or “base” power of 16W. As we move right, the numbers you see represent how much additional power is required by each specific component. For example, next in the sequence you’ll see that 1W is consumed to trickle charge the laptop’s battery. I determined this by first running the laptop without any battery connected. I then installing a charged battery pack and recorded the difference in measured power.
Moving right along the above graph, you’ll see that the T43p’s LCD backlight consumes seven watts. Next, I used the laptop’s power management software to set the CPU to its maximum clock speed, which required an additional four watts (and at this point, no additional software was calling for CPU time). Inserting the optical drive drew another watt. Surprisingly, enabling bluetooth consumed six more watts, while 802.11 wireless only called for 2W. Loading the CPU by enabling World Community Grid called for 10W. Next, running the hard drive with HD Tune required three more watts. Finally, I stressed the graphics card using the OpenGL benchmark within CineBench R10. This drew an additional six watts, bringing the T43p’s maximum power consumption to 56W. Not bad.
Next up, I performed the same tests with my main workhorse, the ThinkPad T61p:
Interestingly, the T61p consumed slightly less base power than the older T43p – just 15W! Its backlight did draw a bit more power at 9W, but this isn’t terribly surprising since its screen is bigger. What I found particularly impressive was the ability of the CPU to throttle back, even when set to its highest clock rate. The difference between an unloaded and loaded CPU at full clock was 22W. The more modern graphics card of the T61p consumed about twice as much power as that of the T43p – fully 13W, bringing the total to 67W. Still not too bad. I should also note that with the T61p there was no noticeable difference in power consumption (e.g. from trickle charging) with the battery removed.
So there you have it! There’s quite a bit of power to be saved by cutting back your CPU speed (if your laptop allows it) as well as disabling your wireless radios and optical drive. Since I’ll be using my T43p as a server, I’ll also be able to disable itsLCD screen, which will get me very close to a base power of 16W. Not quite as good as an some netbooks, or the SheevaPlug, but still not bad given the processing power it affords me.
Oh and if you’re curious, here are my CineBench R10 results for both machines:
- T43p Rendering (Single CPU – Pentium M 760 at 2.0GHz): 1694 CB-CPU
- T43p Shading (OpenGL – 128MB ATI Mobility Fire V3200): 1015 CB-GFX
- T61p Rendering (Single CPU – Intel T9300 at 2.5 GHz): 3061 CB-CPU
- T61p Rendering (Multiple CPU – Intel T9300 at 2.5 GHz): 5757 CB-CPU
- T61p Shading (OpenGL – 256MB NVIDIA Quadro FX 570M): 4212 CB-GFX