Idle Power

In all of these tests you're going to see three charts. The first will show you total platform power, measured at the battery, taking into account everything from SoC to display. The next shows you power measured at the CPU power delivery circuit, and the third shows you power measured at the GPU power delivery circuit. All values are measured in watts, and are reported in 15ms intervals (although I sampled at 1KHz then averaged down to 15ms).

For our first set of tests I simply wanted to get a feel for idle power. Both systems had all background syncing suspended, WiFi was connected, and we're just sitting at the Windows RT/8 Start Screen until the tablets reached a truly idle state. Note that idle under Windows RT/8 technically doesn't happen until the live tiles stop updating, which you'll see denoted by a drop in the idle power consumption in the graphs below.

First up is total platform power consumption:

Surface RT has higher idle power, around 28% on average, compared to Acer's W510. The last half of the graph shows the tablets hitting true idle when the live tiles stop animating.

A look at the CPU chart gives us some more granularity, with Tegra 3 ramping up to higher peak power consumption during all of the periods of activity. Here the Atom Z2760 cores average 36.4mW at idle compared to 70.2mW for Tegra 3.

The GPU specific data is pretty interesting - the GPU power rail shows much high power consumption than on Intel's Z2760. As I didn't design Tegra 3, I don't know what else is powered by this rail - although you'd assume that anything else not in use would be power gated. Imagination Technologies' PowerVR SGX 545 does appear to be quite power efficient here, on average using 155mW while rendering the Start Screen.

I wasn't happy with the peaks we were seeing when nothing was happening on the systems, so to confirm that nothing funny was going on I threw both tablets into airplane mode and waited for full idle. Check out the tail end of the platform power diagram:


That's much better. Without the AP talking to each tablet's WiFi radio constantly, idle becomes truly idle. If you're curious, the power savings are around 47.8mW (average) for the W510 in airplane mode when fully idle.

The GPU rail feeding the Atom Z2760 appears to hit a lower idle power when compared to NVIDIA's Tegra 3. Advantages in idle power consumption are key to delivering good battery life overall.

Introduction OS & App Launch Power
Comments Locked


View All Comments

  • yyrkoon - Tuesday, December 25, 2012 - link

    same OS, different HAL.
  • Krysto - Monday, December 24, 2012 - link

    Good job comparing a last-gen chip like Tegra 3 with Intel's latest Atom chip, Anand. Just because Microsoft is too slow to adopt a cutting edge ARM chip for Windows, doesn't mean Intel is now "toe to toe" with ARM.

    Compare Nexus 10 and iPad 4 with Clover Trail in GPU performance and price, and then let me know how it went.
  • croc - Monday, December 24, 2012 - link

    People that tend to use terms like 'fact' also seem to be such complete tools...
  • Barnassey - Monday, December 24, 2012 - link

    My main thing is why is anand comparing a QUAD core cpu to a DUAL core With hyper-threading cpu? Of course the hyperthreading cpu will use less power.
  • Reikon - Monday, December 24, 2012 - link

    Because the dual core CPU still outperforms the quad core one?
  • Barnassey - Monday, December 24, 2012 - link

    No because the power consumption will be different. Thats the point im trying to make. Plus anand has shown hes a little biased over the years.
  • r1cky4n - Monday, December 24, 2012 - link

    a single atom core is not equivalent to a single a9 core

    The important metric is performance/watt, not watt/core
  • sseemaku - Tuesday, December 25, 2012 - link

    So you want them to disable one of the channels in memory controller and reduce the cache size to match tegra3 for a fair comparison?
  • tempestglen - Monday, December 24, 2012 - link

    It seems that "GPU Workload" and ""Photo Enhance" CPU consumption on summary table are inconsistent with diagrams.
  • tempestglen - Monday, December 24, 2012 - link

    It also seems that Kraken test diagram of total platform power is just same as cpu only diagram. I guess anandtech posted a wrong picture.

    Merry Christmas!

Log in

Don't have an account? Sign up now